If you want population control (or social control, or major social change) you do it so gradually only a few realise what is happening, and they are sufficiently few in number they can be written off as conspiracy theorists.
It seems to me that to allow the economically unproductive to reproduce in greater numbers than the economically productive results in the unproductive representing a slowly increasing proportion of society. When everyone has a vote it's easy to see that once the economically unproductive reach a certain mass any government that promises enough free stuff can pretty much assure its ongoing governance.
Looking around it's clear that "divide and conquer" works very well as a tactic against the people. We see the old and young set against each other, different races set against each other, men and women set against each other, left-wingers and right-wingers set against each other, and all the while those who allegedly represent We The People get to feather their own nests at our expense.