I agree there should be more reflection (moral or factual) into the assumption that we should prioritize preventing human extinction. :)

That being said, we should emphasize that some of the risk factors for extinction also seem to be risk factors for more suffering and s-risks - which suggests that negative utilitarians as well as s-risk reducers wouldn't support shifting focus away from those dealing with those risk factors - unless there are better opportunities for impact. Examples of these risk factors include more conflict, polarization and the unsafe development of AI, especially without concern for cooperative aspects to prevent potential conflict between different AI or their operators.

Of course, this might not apply to all risk factors of extinction. Still, s-risk reducers and suffering reducers might think that it's bad to (intentionally or otherwise) act in a way that results in people trying to bring it about (see https://www.utilitarianism.com/nu/nufaq.html#3.2 ) which might raise the question of precisely how much emphasis to put on this as a community.

More considerations include whether other civilizations exist (e.g. aliens), and if so, how many. This also makes it unclear what antinatalism suggests. If the focus is on fewer births then we need to find out whether human civilization would increase or decrease the total number of births in the future compared to alternative scenarios where, e.g., aliens own the resources humans would have owned.

Also remember that an existential risk (x-risk) is a "risk of an existential catastrophe, i.e. one that threatens the destruction of humanity’s longterm potential". This means existential risks aren't the same as extinction risks. S-risks that destroy humanity's longterm potential are also x-risks.

This question has been considered to some extent by people in the community already. Consider the following posts:

• https://forum.effectivealtruism.org/posts/NfkEqssr7qDazTquW/the-expected-value-of-extinction-risk-reduction-is-positive
• https://forum.effectivealtruism.org/posts/RkPK8rWigSAybgGPe/a-longtermist-critique-of-the-expected-value-of-extinction-2
• https://forum.effectivealtruism.org/posts/cYf6Xx8w7bt9ivbon/which-world-gets-saved

It's would also be worth pointing out that most people in this community who hold views that can be categorized as negative utilitarian or suffering-focused don't endorse bringing about human extinction, e.g.:

• https://magnusvinding.files.wordpress.com/2020/05/suffering-focused-ethics.pdf (chapter 8)
• https://www.utilitarianism.com/nu/nufaq.html#3.2

I am not claiming that these posts/articles have settled the debate, but I think any post on a sensitive topic like this would benefit from including such content.

The big one I can think of, which is related to some of the ones you mention, is leximin or strong enough prioritarianism. The worst off beings human persistence would cause to exist are likely to live net negative lives, possibly very strongly net negative lives if we persist long enough, and on theories like this, benefits to these beings (like preventing their lives) count for vastly more than benefits to better off beings (like by giving those beings good lives rather than no lives). I don’t endorse this view myself, but I think it is the argument that most appeals to me in my moods when I am most sympathetic to extinction. When I sort of inhabit a Tomasikian suffering empathy exercise, and imagine the desperation of the cries of the very worst off being from the future, calling back to me, I can be tempted to decide that rescuing this being in some way is most of what should matter to me.

I don't endorse it, but a-risks could be added: the risks that future human space colonistion will kill or prevent appearance of alien civilizations.

5. Argument from Deep Ecology

    This is similar to the Argument from D-Risks, albeit more down to Earth (pun intended), and is the main stance of groups like the Voluntary Human Extinction Movement. Human civilization has already caused immense harm to the natural environment, and will likely not stop anytime soon. To prevent further damage to the ecosystem, we must allow our problematic species to go extinct.

This seems inconsistent with anti-natalism and negative utilitarianism. If we ought to focus on preventing suffering, why shouldn't anti-natalism also apply to nature? It could be argued that reducing populations of wild animals is a good thing, since it would reduce the amount of suffering in nature, following the same line of reasoning as anti-natalism applied to humans.

Forgive me if what I'm about to suggest is implicit elsewhere, but let's look at what I see as a key premise: humans "being" is a good thing. It's easy to go from there to look instead why humans being may not be a good thing, but why not go the other direction? What if there is A) the being of something else, another form of life/consciousness/whatever, that is a "better being" than "humans being," and 2) the existence of humans is somehow prejudicial or detrimental to the existence and prosperity of that "better being?" This may be something already in existence or expected to be in existence, but with that premise, couldn't you argue that human extinction would itself be a good by perpetuating a better "being?" 

More succinctly put, what if getting humans out of the way gives rise to something better than humans? This can easily devolve into claims of racial superiority within humanity and other assorted BS, but I'm thinking of homo sapiens versus something different (maybe call them neo sapiens, borrowing from what I recall was a mediocre cartoon called Exosquad), either another step on the evolutionary ladder, Skynet, etc.).

2. Negative Utilitarianism

    This is the view that, as utilitarians (or, more broadly, consequentialists), we ought to focus on preventing suffering and pain as opposed to cultivating joy and pleasure; making someone happy is all well and good, but if you cause them to suffer then the harm outweighs the good. This view can imply anti-natalism and is often grouped with it. If we prevent human extinction, then we are responsible for all the suffering endured by every future human who ever lives, which is significant.

Taking that further

It might be that the suffering that would happen along the way to our achievement of pain-free, joyous existence will outweigh our gained benefits. Also, our struggle for such a joyous existence and the suffering that happened along the way might have been a waste because nonexistence is actually not that bad.

Moral presumption

It seems that an argument for moral presumption can be made against preventing extinction. We already know there is great suffering in the world. We do not yet know whether we can end suffering and create a joyous existence. Therefore, it might be more prudent to go extinct.

2. Negative Utilitarianism

    This is the view that, as utilitarians (or, more broadly, consequentialists), we ought to focus on preventing suffering and pain as opposed to cultivating joy and pleasure; making someone happy is all well and good, but if you cause them to suffer then the harm outweighs the good. This view can imply anti-natalism and is often grouped with it. If we prevent human extinction, then we are responsible for all the suffering endured by every future human who ever lives, which is significant.

    3. Argument from S-Risks

    S-Risks are a familiar concept in the EA community, defined as any scenario in which an astronomical amount of suffering is caused, potentially outweighing any benefit of existence. According to this argument, the human race threatens to create such scenarios, especially with more advanced AI and brain mapping technology, and for the sake of these suffering beings we ought to go extinct now and avoid the risk.

    4. Argument from “D-Risks”

    Short for “destruction risks”, I am coining this term to express a concept analogous to S-Risks. If an S-Risk is a scenario in which astronomical suffering is caused, then a D-Risk is a scenario in which astronomical destruction is caused. For example, if future humans were to develop a relativistic kill vehicle (a near-light-speed missile), we could use it to destroy entire planets that potentially harbor life (including Earth). According to this argument, we must again go extinct for the sake of these potentially destroyed lifeforms.

Counterargument that is relevant to all three

We already know that there are many species on Earth, and new ones are evolving all the time. If we let ourselves go extinct, in our absence, species will continue to evolve. It is possible that these species, whether non-human and/or new forms of humans, will evolve to live lives of even more suffering and destruction than we are currently experiencing. We already know that we can create net positive lives for individuals, so we could probably create a species that has virtually zero suffering in the future. Therefore, it is upon us to bring this about.

What's more, the fact that we have such self awareness to consider the possible utility of our own species going extinct might indicate that we are the species that is empowered to ensure that the existing human and nonhuman species, in addition to future species, will be ones that don't suffer. 

Maybe we could destroy all species and their capacity to evolve, thus avoiding the dilemma in the latter paragraph. But then we'd need to be certain that all other species are better off extinct.

I'm a fairly devoted anti-natalist, and I have to say that this ethic has been misunderstood--deeply, woefully so. Firstly, it always has been universal, all sentience included (and so far, it seems that the more humans are born, the less other animals are born). Benatar only focused on our own species because there were reasons to discuss this area in particular. Secondly, human extinction is a purely theoretical scenario, and one that isn't even the core of anti-natalism.

The core is just the negative value of coming into existence. And the basis is suffering reduction by means of prevention. It isn't some cult hellbent on human extinction (even though some of such groups can emerge out there to parasitize on some actual ethics). We wouldn't feel as devastated by the prospects of extinction, but to assume that ending the existence of humanity is our obligation? No, this'd be too much. At least for a contemporary anti-natalist who stays true to the suffering-focused nature of the ethic and is a tad familiar with contemporary SFE research in general (as opposed to someone who is mostly into justifications for their everyday hate). There's this immense ocean of non-human suffering around us. And if not we, then who would take care of it?

We just have our own values. There isn't some magic god thingie in the sky telling us what we should value. There also isn't some sort logical basis that describes the one true morality. Morality only exists as a matter of conscious beings, their thoughts, and their feelings.

So really: I do not care in the slightest if you can come up with a great argument for human extinction. 

If that is what the greatest good is, I will try to kill and destroy those who pursue the 'greatest good'. I value my family, I value my own life. I value the existence of beings like me extended as far and as widely as I possibly can, and I will try to fight anyone who does anything that might endanger that -- and trying to even contemplate this question strikes me as the sort of thing that falls so far out of my Overton window, that the only excuse for not censoring it is that censorship doesn't work.