In general, I don't see how papers which say (little more than) "We agree with X" merit publication. What would be the point of a paper which said, e.g. "We, some utilitarian philosophers, do not think the usual objections to utilitarianism succeed because of the usual counter-objections"? We already know that philosophers believe a variety of things.

I have some sympathy to your general point. However, I think this case is relevantly different from utilitarian philosophers stating they agree with utilitarianism, for the following reasons:

• Many philosophers working in (non-applied) ethics seem to have an attitude of extreme reverence toward Derek Parfit. Parfit rejected the Repugnant Conclusion [1], and essentially founded [2] the field of population ethics on the premise that its task was to find some 'Theory X' that would avoid the Repugnant Conclusion and other problems.
• My impression is that most academic work in population ethics has in fact sought to avoid the Repugnant Conclusion, with papers such as Huemer's In Defence of Repugnance being the exception.
• The name 'Repugnant Conclusion' suggests that it is obviously unacceptable.

None of these claims have true analogs for utilitarianism. It's not the case that the field of normative ethics was conceived as a project to defeat utilitarianism; there is plenty of work arguing for utilitarianism; etc.

More broadly, I think analytic philosophy has a tendency to spawn 'industries' that produce ever more refined attempts and rebuttals of formal theories that try to provide a solution to some problem, the framing of which is usually taking for granted. Perhaps the most infamous examples are countless attempts to find some definition or 'analysis' of the concept of knowledge in terms of more primitive concepts such as justification, truth, and belief, in response to Edmund Gettier's examples allegedly showing that knowledge can't simply be justified true belief. (Indeed, philosophers have discussed the 'Gettier Problem problem', i.e. the philosophical problem of explaining why solving the original Gettier Problem is pointless or otherwise problematic.) Other examples might be the logical positivist project to reduce meaning to predictions of sense data, attempts at defusing van Inwagen's Consequence Argument for the incompatibility of free will and determinism by providing counterexamples to one of its premises, or the ever-growing zoo of Frankfurt-style examples aimed at showing that moral responsibility does not require a 'could have done otherwise' property.

To the extent that there is progress in philosophy, I think it often consists in disrupting such industries by reframing the problem they were built on or forcefully arguing against some desideratum that was thought to be necessary for a 'solution'. (A more cynical view would be that such work merely replaces one flawed industry with the next.) At the very least, such work has often become famous, e.g. Quine's Two Dogmas of Empiricism, Strawson's Freedom and Resentment, Frankfurt's Alternate possibilities and moral responsibility and Freedom of the Will and the Concept of a Person, Kripke's Naming and Necessity, etc.

However, a comparison with such contributions also brings me back to where I agree with you: I think these philosophers provided value because they didn't merely state that they disagreed with, or disliked something about some 'industry'. And, crucially, they even went beyond arguing for or explaining their view. They also made a positive contribution by showing how different and more fruitful work could look like. So e.g., roughly speaking, Quine said "you can't 'reduce' the meaning of an individual proposition to anything, you need to look at the full web of beliefs", Strawson said "the basis for moral responsibility lies not in questions whether or not anyone could have done anything otherwise but in people's 'reactive attitudes' toward each others' behavior", Frankfurt regarding the same issue instead pointed to the internal structure of a moral agent's preferences, etc. Then other philosophers can and did make positive contributions by describing how meaning is a holistic property, what it is about the structure of an agent's internal preferences that makes them morally responsible for their actions, etc.

At least at first glance I couldn't find such a  positive contribution in the paper we're discussing here. It's all well and good to say that one doesn't like the existing population ethics 'industry' - and I agree, in my view the field has been stale for a long time and has consisted mostly of footnotes to Parfit -, but then what else do you want people to do? Quine wouldn't have been nearly as influential had he said "perhaps one day the correct approach to meaning will be uncovered, but I don't know whether Carnap would agree with it". And I suspect the lack of a clear positive recommendation or other 'way out' may prevent this paper from having the effect it tries to have. Though perhaps only time will tell. (E.g. arguably semantic holism wasn't exactly well developed in Two Dogmas itself.)

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[1] At least Parfit clearly rejected the Repugnant Conclusion in Reasons and Persons, Part IV of which seems close to a 'founding document' for population ethics. As the authors of the paper discussed here mention, Parfit seems to later have somewhat changed his stance, though my memory from one of his last papers was still that he was hoping to advance some view avoiding the Repugnant Conclusion (through some combination of lexicality and incomparability or vagueness - indeed the paper was titled Can We Avoid the Repugnant Conclusion?). However, I'm no expert on Parfit's late work and could easily be wrong; e.g. I don't know what if anything he says on population ethics in On What Matters.

[2] There are papers on what we'd today call population ethics that precede Parfit's work, and Reasons in Persons in particular. However, my impression is that Parfit's work, and Reasons and Persons in particular, have had a domineering influence over subsequent work in what became known as population ethics, at least among analytic philosophers in a broadly consequentialist tradition. Again, I'm no expert on the history of population ethics, and would welcome corrections.

Echoing what Max says, I think this paper comes from the assumption that a lot of population ethics is just off down the wrong track of trying to craft theories in a somewhat ad hoc manner that avoid the repugnant conclusion. It is difficult to think of how else these people could try and make this point given that making the same points that others have made before, in some cases several decades ago, would not be publishable because they are not novel. This strikes me as something of a (frustrated?) last resort to try and make the discipline acknowledge that there might be a problem in the way it has been going for thirty years. 

I suppose one alternative would have been to publish this on a philosophy blog, but then it would necessarily have got less reach than getting it in a top journal. 

Although unusual in philosophy, the practice is widespread in science. Scientists will often write in short letters criticising published articles that are light on substantive argument but reiterate a view among some prominent researchers. 

Finally, I think it is useful to have more surveys of what different researchers in a field believe, and this is one such instance of that - it tells us that several of the world's best moral philosophers are willing to accept this thing that everyone else seems to think is insane. 

I received a nice reply from Dean which I've asked if I can share. Assuming he says yes, I'll have a more thought out response to this point soon.

Here are some quick thoughts: There are many issues in all academic fields, the vast majority of which are not paid the appropriate amount of attention. Some are overvalued, some are unfairly ignored. That's too bad, and I'm very glad that movements like EA exist to call more attention to pressing research questions that might otherwise get ignored.

What I'm afraid of is living in a world where researchers see it as part of their charter to correct each of these attentional inexactitudes, and do so by gathering bands of other academics to many-author a paper which basically just calls for a greater/lesser amount of attention to be paid to some issue.

Why would that be bad?

1. It's not a balanced process. Unlike the IGM Experts Panel, no one is being surveyed and there's no presentation of disagreement or distribution of beliefs over the field. How do we know there aren't 30 equally prominent people willing to say the Repugnant Conclusion is actually very important? Should they go out and many-author their own paper?
2. A lot of this is very subjective, you're just arguing that an issue receives more/less attention than is merited. That's fine as a personal judgement, but it's hard for anyone else to argue against on an object-level. This risks politicization.
3. There are perverse incentives. I'm not claiming that's what's at play here, but it's a risk this precedent sets. When academics argue for the (un)importance of various research questions, they are also arguing for their own tenure, departmental funding, etc. This is an unavoidable part of the academic career, but it should be limited to careerist venues, not academic publications.

Again, those are some quick thoughts from an outsider, so I wouldn't attach too much credence to them. But I hope that help explains why this strikes me as somewhat perilous.

Once shared, I think Dean's response will show that my concerns are, in practice, not very serious.

That's a good way of framing it. I absolutely agree that individuals and groups should reflect on whether or not their time is being spent wisely.

Here are some possible failure modes. I am not saying that any of these are occurring in this particular situation. As a naive outsider looking in, this is merely what springs to mind when I consider what might happen if this type of publishing were to become commonplace.

• Imagine I am a mildly prominent academic. One day, a colleague sends me a draft of a paper, asking if I would like to co-author it. He tells me that the other co-authors include Yew-Kwang Ng, Toby Ord, Hilary Greaves and other superstars. I haven't given the object-level claims much thought, but I'm eager to associate with high-status academics and get my name on a publication in Utilitas. I go ahead and sign off.

• Imagine I am a junior academic. One day, I have an insight that may lead to an important advance in population ethics, but it relies on some discussion of the Repugnant Conclusion. As I discuss this idea with colleagues, I'm directed to this many-authored paper indicating that we should not pay too much attention to the Repugnant Conclusion. I don't take issue with any of the paper's object-level claims, I simply believe that my finding is important whether or not it's in an subfield that has received "too much focus". My colleagues have no opinion on the matter at hand, but keep referring me to the many-authored paper anyway, mumbling something about expert consensus. In the end, I'm persuaded not to publish.

• Imagine I am a very prominent academic with a solid reputation. I now want to raise more grant funding for my department, so I write a short draft making the claim that my subfield has received too little focus. I pass this around to mildly prominent academics, who sign off on the paper in order to associate with me and get their name on a publication in Utilitas. With 30 prominent academics on the paper, no journal would dare deny me publication.

Again, my stance here is not as an academic. These are speculative failure modes, not real scenarios I've seen, and certainly not real accusations I'm making of the specific authors in question here. My goal is to express what I believe to be a reasonable discomfort, and seek clarification on how the academic institutions at play actually function.