Well many countries are already there. They don't plateau. They just keep crashing.

Don't, life is beautiful and bringing more of it into the world is a good thing. Besides, overpopulation isn't the problem anymore, it's aging. More kids will help with that problem.

There are, but Namor and his people would not be since since they are of purely pre-Columbian Mayans that cut themselves off from the outside world and thus didn't have the introduction of European genes that the rest of the Americas would receive over the next few centuries.