I don't think that's even close to universally true. I know quite a few vegetarians (and vegans) who were "doing it right", but were later advised by their doctors (even after getting 2nd and sometimes 3rd opinions) to eat meat as a means of curing their systemic health problems. And guess what? Eating meat made them healthy.

I'm not saying that there aren't tons of people who are very healthy vegetarians and vegans (some of them healthier than they would be eating meat), but I'd just like to point out that it's not universally true that you can replace animal protein with plant-based protein and be healthy. Nutrition is a complex topic, and not all bodies behave the same when it comes to food.

Some people can. Some people find that the phytochemicals associated with those plant proteins, when consumed in quantities, cause significant gut and hormonal problems (phytoestrogens impacting androgen production, for example). And that says nothing about the current questions about whether soy increases breast cancer risk.

Soy is obviously not the only plant protein. My current favorite is mushroom-based, but it is new, and industrial food doesn't have a good track record.