American diets are obscene, but in terms of carrying capacity and ecology, a mildly omnivore diet still feeds more people than a vegan diet, due to the presence of land that is unsuitable for cultivation, in-between periods of dry/cold weather where crops struggle to grow, etc
Also consider the pests that cropland attracts, which under a holistic pre-colonial agriculture mode (which produces more calories per acre at the expense of more human labor required), would have often been dealt with through killing (and eating)
Medieval animal use patterns are way greener than what we have now. Pigs acted as garbage disposals, then by day a professional swineherd would take everyone's pigs out to the forest. Cows and sheep ate grass off useless rocky land, and were walked to where they'd be slaughtered.
But I don't really think this is a deep argument for how things should be today. How can you restructure cities so pigs can commute between homes and the forest? How can cows walk all the way to pasture and back to the city for slaughter? I don't think it's actually possible; the density of modern cities breaks these models. But that same density is ecologically required a thousand other ways.
if you force rich people to use their stashed-away wealth to fund this stuff then it's a non-issue
everyone in NYC already eats pork, they can eat pork the same way they do now. Animal agriculture only accounts for 6% of emissions total (even when you include these dumb unhealthy beef-guzzling white males) while transportation is 30%
if that's even true it could only be relevant in a world where there's not enough agricultural land to support growing plants fit for human consumption, which is not the world we live in
getting everyone to eat the algae is not going to be fun, but it would be an excellent backup to send to food scarce regions. Although the thought of just shoving some light greens at people instead of just giving them regular food seems shitty