I wonder if the cutoff point is more like 25,000 though, the number of broiler chickens raised in a shed. It's unclear to me whether producers respond to small changes in demand by adjusting the numbers of broilers in a shed or only by adjusting the number of sheds in use.

If the cutoff point is more like 25,000, then this would imply that most veg*ns go their entire lives without preventing the existence of a single broiler through their consumption changes, while a minority prevent the existence of a huge number.

For what it's worth, it seems likely that donations to AMF are similar since their distributions typically cover hundreds of thousands or millions of people.

Is there other evidence supporting or falsifying Will's position from another source.

Following a preference utilitarian system, you are correct. Hare discusses this in, Why I Am only a Demi-vegetarian. Singer also mention it in, Singer and His Critics.

Although, that's not the reality today (in the US at least). Unethical living conditions, such at battery cages for chickens or a short life confined to small pens for other livestock is the point. No such being wants to suffer unnecessarily. On the other hand, if factory farming was like ol' MacDonald farm, then sure. Kind of a paradox...

The idea behind trying to end factory farming for animals' sake is that animals who spend their whole lives on factory farms are enduring lives that are not worth living. It is better not to bring creatures into existence who would live net negative lives.

You're right that extinction is a (very) extreme case. It's more likely that even with a drastic reduction in factory farming, a small fraction of descendants of farmed species would be preserved--either for farming, or in zoos or similar institutions. After all, they're easy to domesticate, having been bred over the centuries for precisely those purposes.

That some chickens will never be born at all is the goal, as:

1. it's believed those chickens born would have lives of only suffering, not redeemed by happiness;

2. the degree and constancy of the suffering is so great considerations of preferences the chicken may have, like a 'will to live', are overridden by the preference/desire to be free of suffering;

3. the expected consensus is we know enough about animal minds to conclude they have preferences like the instantaneous desire to be free of suffering in any given moment, but we don't have sufficient reason to believe they abstractly think of the future, and meaningfully have a 'will to live';

4. the collective experience of the farm animal rights movement has been decades of reforms of factory farms remain unenforced or are insufficient to overcome the above considerations about how the lives of chickens on factory farms will never be worth living.

So the goal of some effective altruists focused on present and near-term future non-human animal well-being isn't to advocate for the animal's rights so much as it is to mitigate factory farming as an industry. This is from a perspective of EA from years ago, when Doing Good Better was published. There has been an empirical revolution within effective animal advocacy since then. The evidence has borne out employing messaging focused on systemic change over individual dietary/behavioural change, and not splitting hairs in messaging based on ideological differences internal to the animal welfare/rights movement. So if one cares about the rights of species to not go extinct, one doesn't have to fear the movement strategy implied by the OP, as effective animal advocacy (EAA) organizations are mostly not pursuing that strategy anymore. Given how expansive factory farming is in developed Western countries, and how it's expanding in developing countries, it appears factory farming, and thus the species of farm chicken, isn't going away soon. That stated, I've no reason to think effective animal advocates would object to preserving the genome of the farm chicken, or rearing individual farm chickens under humane conditions, e.g., at an animal shelter or hobby farm.

Of course peers of EA outside the movement have weighed on the topic, disagreeing with the consensus EA position on either side. An argument against vegetarianism and for the continuation of factory farming exists in the logic of the larder, as laid out by Robin Hanson and others. On the other side, another animal liberation movement called Direct Action Everywhere (DxE) thinks EAA doesn't go far enough. While I haven't followed them closely, and so I find their end goals confusing, I believe DxE's strategy is to mitigate factory farming isn't to have them not be born into net-negative lives, but raising sufficient public consciousness global human civilization will at some point in the future literally directly liberate all presently factory-farmed animals, presumably to freely roam the Earth.

*"effective animal advocacy" is the term for the interstitial movement emerging from the combination of effective altruism and the conventional animal welfare/rights movement.

I get it pretty frequently from newcomers (maybe in the top 20 questions for animal-focused EA?), but everyone seems convinced by a brief explanation of how there's still a small chance of big purchasing changes even if every small consumption change doesn't always lead to a purchasing change.

I think the economist guesses are from Compassion, By the Pound, though I also don't have a copy of either book.

no peer-reviewed articles or analyses being quoted

Yeah. Matheny (2003) is a journal article on the same topic, though it's not an economics journal.

they could easily export whatever is not locally consumed (as some EU countries do).

Perhaps that would reduce local meat production in the destination countries.

I took this to mean "even if you don't expect your choice to have economic impact, like your friend ordered the KFC bucket but doesn't want to finish it and asks if you'd like some, there are still other factors to consider like norm-setting and your own cognitive dissonance."

I'm referring to effective altruists who aren't (yet) veg-n but are considering becoming so, and are open-minded about but currently unconvinced by the argument veg-nism has a genuine economic impact.