One of the biggest flaws of democracies is that they strongly incentivize policy makers to follow short-term gains over long-term goals.
The USSR planned its world in 5-year spans. That's about the same length of time between elections in most democracies. Lots of people like to say things like "one of the biggest flaws of democracies" but I've yet to see such a statement that actually doesn't also apply to other systems of government.
Realistically population one of the few things you can take a long term view on, since so geo politics and technology are so unpredictable. With population planning much of the effects are only to come a generation later so mostly only a long term concern. It is also completely radioactive as far as modern democracies are concerned.