And the earth will still be overpopulated.
The decline in birthrate is a self-correcting way of bringing human population back to its natural balance in nature.
Because of overpopulation, we've had to develop technology to make more of less, but throughout we ignore man's evolved nature, which is not in accord with modern living.
This conflict of man's nature with his modern artificial environment, works to depress his urge to reproduce, hence the declining population in fully industrialized societies.
Your opinion is wrong. Your post sounds like something that would come out of a gaia-worshipping, Malthusian pagan.
A little thought experiment - two nations side by side. One in which the people think like you and one in which the people reject Malthusian arguments. Since you like to be a man of “Reason”, which nation will dominate the other within 100 years?
Makes sense. There's nothing sexy about growing up in a rat cage, then going to work in a cubicle and then retire in an isolated fortress.