I think a major part of why people lapse completely is that there exists a widespread belief that veg*nism is a hard and fast label, and falling off the wagon is irreversible. Many ex-vegans I know now eat completely omni diets that are simply at odds with their moral beliefs. I think that promoting the idea that 80⁄20 veganism is okay will result in a lot less animal product consumption overall. This article explains the way this black-and-white thinking negatively affects peoples’ beliefs around their diet and personal identity.
Religious communities have a lot to teach us here. Picture the following recruitment strategy:
target the majority of a population in a small, stable area (liberal upper-middle class suburbia seems like a good initial candidate)
focus on recruiting the influencers and the household cooks
hold weekly or monthly food-related events (communal meals) where people have an opportunity to learn more, question, make public commitments and be supported by their peers
promote adherence to values rather than strict rules, and encourage people to honestly discuss their challenges and successes.
Eventually, this kind of community could function like a local atheist spiritual community of some kind, and be a platform for spreading other effective altruism ideas.
In most parts of the world, veganism is not the norm, which I believe is a big part of why people find it so challenging long-term. Focusing on universities strengthens the perception that veganism is merely part of a normal period of experimentation. Once there are people who live in communities where more than 50% of people are veg*n, I suspect it will be much easier to both retain those people and convert new people.
I use a harm-minimisation strategy when I’m in food deserts or traveling, but I suspect I’m very rare. For many vegans I know, not eating any animal products at all is part of their identity, so being forced to consume them because of a lack of availability would be incredibly emotionally stressful for them and create a lot of cognitive dissonance.