All people should eat the diet they are constitutionally suited for.
However, I wanted to take issue with this statement of yours:
"Have you ever met any super grumpy, unhealthy vegetarians?"
Yes, there's plenty of them.
There's also a lot of super grumpy, unhealthy meat-eaters.
When vegetarians get sick with Disease A or B or C, it is attributed to their lack of meat. Okay.
But when meat eaters get those same symptoms and those same diseases, diet is never the cause, is it.
Maybe it isn't all about diet, after all. Maybe there's a dozen different factors. Maybe we have to look at the totality of the individual and forget our biases one way or another.