1. "healthy" doesn't mean living a long life free of ailments. It means achieving your standard of athletic performance as a hobby.
2. "challenging" doesn't mean difficult. It means doing some research, buying more tempeh at once and maybe freezing it, or just buying a protein supplement.
That's already too much. I can do that, and you can do that. Our jobs are to research things and then implement them. Most people will not or can not do this. Expecting them to is a waste of time.
The way to solve this would be to de-incentivize meat production on the supply side, and encourage food distributors to fortify food with micronutrients that are commonly missing in vegetarian diets (vitamin B12, calcium, etc). Anything short of this is not going to do much except turn people off from being vegetarian.
I'm still learning this stuff myself. But from the things I've read so far, vegans (especially the WFPB variety) can have difficulty getting: Vitamin B12, calcium[0], iron, and possibly a balanced amino acid profile if they have elevated protein requirements. A diet too high in carbs and too low in fat and protein can also cause long term problems like candida overgrowth and hormonal imbalances.
I agree that none of this matters as long as people can eat meat. They will if they can.
Yes, that's a decent summery. Populations would be less healthy if meat just suddenly disappeared with no other changes. Although, in the United States at least, we massively over-consume meat to the detriment of our health. But we rely on it as a source of many micronutrients that we otherwise wouldn't get by default.
Additionally, these changes will never happen without regulatory changes on the supply side to artificially drive up the cost of meat production.
Also bear in mind that not all protein is created equal. If wheat gluten wasn't low in lysine, I'd eat a lot more of it.