The carnist here points to some vaguely worded surveys from social media sites as proof that veganism is not long-term sustainable at the top of their "evidence-based copypasta".
So this is the line-one item for "anti-vegans". Their main piece of evidence regarding veganism and health is an uncontrolled, self-reported, vaguely worded questionnaire spread over social media with no verification of participants, controls for placebo, etc.
One could equally make an anecdotal questionnaire for anything. And I'm confident in the 1950s you could have found many more people that smoke (or relapsed after quitting) than managed to quit for good. What would that mean about smoking that "former non-smokers" outnumber "non-smokers" by some survey? Or what does it mean that only a quarter of people that "quit heroin" manage to stay clean? Former quitters outnumber quitters 3:1 for heroin. Does that mean that heroin contains some vital nutrient that you simply cannot live without?
I tried a vegan diet one summer before returning to vegetarianism (before I became a full-time vegan). Did that make me an ex-vegan? I mean I didn't do it for ethical reasons. Did the people that cited "boredom with veganism" as a reason to leave actually suddenly decide "I'm bored avoiding animal cruelty"? It makes no sense. It means veganism wasn't well-defined for the study.
Why are there so many plant-based products on shelves and fast food joints providing vegan options? Are they just spending money making food that no one buys? That doesn't make sense.
Who cares if there are 10, 100, or a million times as many ex-vegans than vegans? If it stands on moral grounds then it stands on moral grounds, and if you can do it you therefore should. Who cares what other people are doing?