Some people will want to have large families, but on average it seems that most people don't.
It may not be the case that this is how it will work, but it's not a foregone conclusion that "on average" most people will continue to not want to have kids even in the "first world."
(Don't Mormons for instance have a fertility rate well above replacement despite also having higher than average incomes?)
There is pretty strong evidence that the variance in the number of offspring is about 50% genetic.