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.
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.