“post-scarcity” in this context doesn’t mean “everyone gets everything they want whenever they want it”. Maybe I want to own a planet, but there aren’t enough planets to go around, and nobody actually believes in a future where everyone can get their own planet.
When talking about these things, it’s best not to assume the most ridiculous interpretation of what the other person is saying. e.g instead of reading “post-scarcity” to mean “everyone gets everything all the time no matter what”, read it to mean “everyone gets what they need”.
also for what it’s worth, I’ve been an ethical vegan for several years after being a die-hard meat eater and literally convincing people close to me to move away from veganism/vegetarianism exactly for health reasons (I had the same misconception you did about veganism). After actually going vegan, doing absolutely no meal planning, no exercise, no calorie counting, still eating mostly frozen food and pickup, my blood pressure as a lean 6’1 mid 20s male has gone from pre-hypertension to normal levels. I get my blood checked regularly and I’m far healthier than I was when I was downing popeyes, jersey mikes, and five guys several times a week. And I’m not just eating salads or whatever, I’m usually having vegan buffalo “chicken” or beyond burgers.
I don’t advocate veganism based on health benefits (veganism is an ethical philosophy), but vegan diets are baseline much healthier than the baseline for non-vegan diets. You can’t go as wrong with them as the vast majority of Americans do with their diets.