'What if' is pointless. What if vegetarianism makes you stronger than eating meat? What if it increases your IQ by 20 points or makes you live 200 years? What if you can code faster drinking rare pygmy tree sap or the blood of certain albino poison toads?

> you can't prove that it's actually healthier, because the current state of dietary science is pretty poor.

Almost every decision in life must be made without proof, but with evidence and judgment. We know a lot about nutrition, and a lot of evidence points toward health benefits in eating more vegetables and less meat. We can also see lots of vegetarians in our communities and they don't seem sickly or shorter, etc. - we also see elite athletes in public who are vegetarians.

> a diet consisting of vegetables

Vegetarianim is much more than vegetables; it's everything but meat - legumes (generally beans), vegetables, fruits, grains, nuts - plus eggs and cheese. Vegans cut out the latter two items.

> What if being vegetarian makes you smaller and weaker physically (perhaps the case in some vegetarian countries now).

Where?

> evolutionary problems that could come from some humans going vegetarian while others don't.

What problems? How does diet affect evolution? We'll lose our hunting muscles over the next 500,000 years? Remember humans haven't changed much biologically in 200,000+ years.