There’s no practical downside?

inconvenience, social ostracization, and aesthetic deprivation are actual downsides. further you don’t actually know what is good for them: you’re not their doctor. finally, it would be better for the animals if it shrunk industry. the industry grows every year, so it is not any better for the animals.

why not do the little extra step of going vegan?

because it’s ineffective at addressing the issues facing the environment.

Animal agriculture accounts for 40-60% of total emissions.

all of agriculture is only about 20% of our emissions, so this just can’t be true.

veganism is quantifiably healthier and correlated with longevity.

in some respects for some people. some of the studies (china study, for instance) have been debunked, though.

you aren’t a dietician. even if you are (i admit, i was guessing), you aren’t my dietician, so maybe you shouldn’t be telling other people what a healthy diet would be for them.