I was a very strict vegan for 6.5 years (going as far as avoiding palm oil also, buying fair trade bananas, coffee, chocolate, and strict in nonfood aspects) but am now vegetarian as far as diet goes, while maintaining vegan standards as far as nonfood items (and no I do not eat fish lol).
Quite a few people do not stay vegan but many of those people do maintain some sort of decrease in animal products. My sister was vegan for two years but is now mostly vegetarian but very occasionally eats meat when eating out (which is rare for her). My Dad is a meat eater but drinks almond milk exclusively and avoids dairy (due to digestive issues). There were 118 vegans in my local Vegan Meetup group (which has now disbanded due to lack of leadership) but a few of them said they were strict vegetarians, not vegans (still wore old leather or didn't avoid ingredients like honey or chose vegetarian options when eating out if there was nothing vegan).
I went to the
Vegan Butcher shop in Minneapolis about a year ago and was told that 50% of the customers that went there were meat eaters, so the interest in plant based products is not limited to vegans.