Well no shit. That applies to most animals we humans care for, even the ones who we don’t typically eat. Try throwing a hairless cat or a pug out into the wild. They can’t manage without us no more.
Interestingly enough you don’t have to be so specific as Black Angus. All cows are totally extinct in the wild. They derive from the Eurasian auroch which went extinct in most places of its original range over 3000 years ago. The absolute last one died in 1627 in Poland, but even that one was probably not pure auroch. If everyone went vegan we would probably still keep a few cows around in zoos but we would have no where near the amount we have today. If we wanted to reintroduce something similar we would have to rely on reintroducing european buffalos, which are another species but still native to Europe.









In the US the native herbivore with the “cow-niche” is the American bison. If we would restore ecosystems and replace captive grazers with wild grazers, increasing the wild bison population is the answer and much preferable to having wild cows (who don’t even exist in the first place, the wild version is extinct as mentioned). Of course bison is not an answer to what to do with the cows that already exist in the US of course.
However if a decision was made to ban all animal agriculture I would be a strong opponent of not rewilding any cows. They are not native and they are not even fit for living in the wild anymore. Just take a Holstein milking cow for example. What use does producing 40liter of milk per day have in the wild? None! Calves can’t drink even close to that amount. The lactating moms would get mastitis. They are not even fit to only make milk for just their calves anymore. Let the domestic cows die out in that case.