This is a pretty poor explanation for people stopping eating vegan. You could apply the same argument if the average time someone remained vegan were anywhere from 2 months to an entire lifetime.

If you value humans according to their brain mass and donate to human-focused charities, you should only donate to charities that benefit adult males since they have more brain mass than adult females and certainly than children.

It's much more compelling by comparison, especially when backed up with the fact that the person at issue actually does care about all other beings, including animals (even if s/he doesn't believe veganism is the most effective possible way of reducing suffering).

Not really, if you believe so you should cite studies showing that vegetarians are less happy than non-vegetarians on average. Your preferences adapt (hedonic treadmill, etc.). Furthermore, abundant research shows that vegetarians live several years longer than non-vegetarians and are healthier on average.