What are your plans for all the currently living domesticated animals if, hypothetically, meat eating is made illegal?

I haven't here advocated for making meat-eating illegal. If nothing else, at the current moment, that's infeasible for a number of reasons, and even if it became mostly feasible, there would probably always have to be some exceptions (e.g., people who have very specific dietary requirements, although maybe lab-grown meat could plug that hole?).

That said, thinking purely hypothetically, I recognize two likely endgame scenarios.

1. A gradual phasing out of animal agriculture. Suppose legislation is passed that increasingly limits animal agriculture over the course of, say, twenty years. Animal stocks dwindle over time, eventually being reduced to nothing.
2. A mass holocaust, akin to what Denmark did to their mink stocks in 2020. This sounds horrible, but it is, ethically speaking, actually the better option, because it results in the smallest amount of total suffering (i.e., the area under the daily suffering curve is greater in Option 1 then in Option 2).

Have you ever considered that being raised by humans for consumption is literally the most wildly successful species survival strategy that natural selection has ever thrown up?

This is completely irrelevant. For me, veganism is basically just what happens when you take utilitarianism and extend it to include the experiences of non-human animals. I care about individuals. I don't care one whit about species per se.

Meat is one of the most nutrient dense foods out there and is likely the entire reason we were able to develop these incredibly energy and nutrient expensive brains, have you considered what the long term species ramifications are for us if we choose to stop a standard practice that has been with us since before our species was even human yet?

Do you not think the critical need for specific supplements to maintain good health is a sign that the diet was never intended for our normal operation?

I'll take both of these at the same time, because my thoughts on them are basically the same.

We were not designed by a god. We were not "intended" for anything. Evolution has no normative value. To believe that it does is pseudoscience (or, perhaps, pseudo-philosophy).

People who argue that veganism is "unnatural" are arbitrarily picking out one out of the innumerable ways that the lives of humans today differ from those of the past. If I suggested that we ought to revert to being subsistence hunter-gatherers in Africa living in groups of ~100 people, you would call me insane. So the mere fact that something is different from the conditions in which we evolved means absolutely nothing.

The question is simply this: can we reduce suffering? If we can, we should, regardless of how "unnatural" the solution is.

If you can provide me a scientific argument against veganism in principle, that would be worth considering. Merely gesturing at the need for supplementation says nothing to me. If it works, it works.

What is your stance on pets?

I haven't figured this one out for myself yet. I think the anti-pet people have compelling arguments, and I have a lot of cognitive dissonance over that fact.

I would like to hear your opinion on parents raising their infants to be vegan from birth.

This one I'm not sure about, at least right now, simply due our lack of knowledge. My guess is that it's theoretically possible to raise an infant as a vegan without any problems, but that it's more difficult to do it right. I don't know if I'd trust myself to do it. I think this is a problem that will require a lot of studies to figure out, but I also think it's worth figuring out.

What are your plans for all the currently living domesticated animals if, hypothetically, meat eating is made illegal?

I am a vegan. Is this conversation unreasonable?

Are you talking to the same person, or the same few people, repeatedly? There certainly are people out there who just are unreasonable. You can't expect individuals to change.

Otherwise, I guess (and I admit that this is biased in my favor) that you simply disagree with each other at a foundational level, and that's causing you to talk past each other.

I think that most people don't really know how to discourse with people who have differing ethical foundations, because it can lead to situations where a person who meets all the societal criteria of a "good person" is nonetheless committing (according to whatever ethical precepts) a horrible crime. But, in this context, accusing someone of committing a horrible crime is not unreasonable; in fact, it's too reasonable; it involves prioritizing reason over tact and politeness.

I'd be interested in seeing a trace of one of those conversations.

For the most part, the "unreasonable vegan" stereotype comes from two places.

1. Confirmation bias. Veganism makes people uncomfortable with their own decisions, so people spread around the most outrageous stories about vegans as a defense mechanism. This is the same thing that happens in various circles with anyone whose mere existence makes other people insecure; e.g., teetotalers, or polyamorous people.
2. Just plain disagreeing with them. There are lots of vegan arguments that are logically valid, but they sound outrageous if you don't already agree with them. People have trouble looking past their initial emotional reactions, so they respond to logically valid arguments with mere incredulity.