People may indeed limit themselves to having only two, one, or even no children at all, but that will only lead to extinction of such self-limiting groups. Besides these, there also are people that see nothing more important (but not necessarily better) than leaving behind their own kind as offspring. Unless some effect kicks in, like a social drive that instill in the masses the idea of breeding less as it's in western culture, or a government-enforced program to artificially control the demographic dynamic as in China, these kind of people will prevail in the long run. And that's a good thing, I think.