The gut bacteria thing might make sense, but a lot of people transition. They would then have the new biome. Extra farting and all.
I think it's a culture issue, as in the culture teaching you what works and what does not, etc. in terms of eating habits and recipes. Maybe once a robust culture forms around eating vegetarian food where you live, then maybe all the doubts might go away.
I don't know, it's an educated guess. I know they aren't richer in India, and they are doing fine.