Everybody Trades

In AVENGERS: INFINITY WAR, Thanos thinks he can "save" the universe by killing half the living and/or intelligent beings in it. First of all, he's wrong. But even if his premise were correct, doesn't "half" seem pretty arbitrary? "You don't know that!" is the quote of the film. If overpopulation is going to doom humanity, then why are we better off today with far more people than ever?