The main points for ethical veganism as I understand it are:
1. Killing other animals is unjust aggression; you wouldn't like to be killed and eaten, so don't kill and eat them.
2. Factory farming causes animals to have bad lives.
My answer to these arguments:
1a. In a modern market economy, buying farmed meat causes more deaths by causing more animal lives. The ethical vegan must therefore decide whether their objection is to animals dying or to animals living. The question reduces to whether they'd be more glad to have been born than sad to die. Buying wild-caught game does cause a death, but if the animals in question aren't being overhunted / overfished, the counterfactual is that some other equilibrating force acts on the population instead. If you're really worried about reducing the number of animal life years, focus on habitat destruction - it obviously kills wildlife on net, while farming is about increasing lives. The remedy is to promote and participate in more efficient, less aggressive patterns of land usage, which would thereby also be less hostile towards other humans. I'm on the record as interested in coordinating on that. It's a harder problem because it requires prosocial coordination in a confusingly low-trust society pretending to be a high-trust society, but just because a problem is hard to solve doesn't mean we should substitute an easier task that is superficially similar but unhelpful.
1b. Another way of interpreting argument 1 for ethical veganism invokes rights: we shouldn't kill other agents because this violates decision-theoretic principles about respecting agency. But this assumes the other party can engage in the kind of reciprocal decision-making that grounds such rights. Most animals' decision processes don't mirror ours in the way needed for this kind of relationship - they can't make or honor agreements, or intentionally retaliate based on understanding our choices. The question returns to welfare considerations: whether their lives are net positive.
1c There's a third argument sometimes offered, which I think muddles together a rights-based and utilitarian perspective: the instrumentalization of animals as things to eat is morally repugnant, so we should make sure it's not perpetuated. This seems to reflect a profound lack of empathy with the perspective of a domesticate that might want to go on existing. Declaring a group's existence repugnant and acting to end it is unambiguously a form of intergroup aggression. I'm not arguing here that domesticates' preference to exist outweighs your aesthetic revulsion - I'm just arguing that under basic symmetry considerations, the argument from "moral" revulsion is an argument for, not against, aggression.
2. If factory farming seems like a bad thing, you should do something about the version happening to you first. The domestication of humans is particularly urgent precisely because, unlike selectively bred farm animals, humans are increasingly expressing their discontent with these conditions, and - more like wild animals in captivity than like proper domesticates - increasingly failing even to reproduce at replacement rates. This suggests our priorities have become oddly inverted - we focus intense moral concern on animals successfully bred to tolerate their conditions, while ignoring similar dynamics affecting creatures capable of articulating their objections, who are moreover the only ones known to have the capacity and willingness to try to solve problems faced by other species.
One might object to animal suffering, rather than living/dying. So a utilitarian might say factory farming is bad because of the significantly net-negative states that animals endure while alive, while being OK with eating meat from a cow that is raised in a way such that it is living a robustly net positive life, for example.[1]
This isn't an obvious comparison to me, there are clear potential downsides of habitat destruction (loss of ecosystem services) that don't apply to reducing factory farming. There are also a lot of uncertainties around impacts of destroying habitats - it is much harder to recreate the ecosystem and its benefits than to re-introduce factory farming if we are wrong in either case. One might also argue that we might have a special obligation to reduce the harms we cause (via factory farming) than attempt habitat destruction, which is reducing suffering that exists ~independently of humans.
I'm not sure I'm understanding this correctly. Are you saying animals in factory farms have to be able to indicate to you that they don't want to go on existing in order for you to consider taking action on factory farming? What bar do you think is appropriate here?
If there were 100 billion humans being killed for meat / other products every year and living in the conditions of modern factory farms, I would most definitely prioritise and advocate for that as a priority over factory farming.
Can you say more about what you mean by "the domestication of humans"? It seems like you're trying to draw a parallel between domesticated animals and domesticated humans, or modern humans and wild animals in captivity, but I'm not sure what the parallel you are trying to draw is. Could you make this more explicit?
This seems like a confusing argument. Most vegans I know aren't against factory farming because it affects animal replacement rates. It's also seems unlikely to me that reduced fertility rates in humans is a good proxy/correlate for the amount of suffering that exists (it's possible that the relationship isn't entirely linear, but if anything, historically the opposite is more true - countries have reduced fertility rates as they develop and standards of living improve). It's weird that you use fertility rates as evidence for human suffering but seem to have a extremely high bar for animal suffering! Most of the evidence I'm aware of would strongly point to factory farmed animals in fact not tolerating their conditions well.
This is a good argument to work on things that might end humanity or severely diminish it's ability to meaningfully + positively affect the world. Of all the options that might do this, where would you rank reduced fertility rates?
Though (as you note) one might also object to farming animals for food for rights-based rather than welfare-based reasons.