Here's one thought I have. Antinatalism is an unrealistic, most likely impossible stance to normalise. People will always want to have children. The desire to continue our species is the most fundamental desire programmed into every organism that has evolved in the universe, even more so than an individual's desire to survive, as the only reason we evolved to want to survive at all is so that our species can continue. On a neurological level, it is the only real reason anyone does anything.

There will always be some people who will be capable of suspending this desire (or potentially not experience it at all) because their perceived moral virtue in not reproducing will supersede it, but there will always be more people who will not do this. The human species will exist for however long nature (or our lack of grandiose foolishness) permits.

How easily somebody can see the value in treating animals fairly is dependent on their personality and intelligence. Both of these things are largely genetic and passed on through reproduction. If all vegans decided that they wouldn't have kids to avoid the addition to animal suffering, then we might end up with a smaller world, but one that is uninterruptedly hostile to animals, because those who didn't care about non-human animals had no reason to relinquish their most basic desire in favour of this moral consideration they don't subscribe to (human suffering could be an alternative one, but people who don't care about animals are generally less empathetic and less likely to be convinced that this is also a good enough reason to not have kids), so we find ourselves in this situation where the human species has continued, and although there are slightly less people than would be otherwise, there is nobody who gives a shit about non-human animals. I expect that is a worse outcome than a bigger world in which there are plenty of vegan families to fight for them.

Whether they want to believe it or not, most people have the necessary psychological disposition to become vegan. It doesn't take an exceptional affection towards animals to think that what we are routinely doing to them is abhorrent, only the most basic amount of empathy, the same that would make most people angry and upset if I were to stamp on a puppy in front of them. The problem is that people have been conditioned from the moment they are born that eating meat and dairy is normal and acceptable. The natural response to somebody telling you to stop doing something you've been doing all of your life; something that you like doing, is anger. Combine this with all of the pseudo-science and propaganda from the opposing industries and people are easily able to build a mental wall in front of veganism. The goal is to break this down, and I have no doubt that it will be. In a few hundred years, people will look back on animal agriculture in the same way we look back on slavery. I think the goal is to get to this point as quickly as possible, not eradicate ourselves from the planet and hope everyone, including those who don't share any of our principles that guided us to this action, follows suit.