Education? Contraception? It’s not fucking rocket science. Every developed country in the world is at well below replacement rates. The idea needs to be promoted and not derided or conflated with eugenics (which it emphatically isn’t). Blunting the impact of an aging population is the most difficult problem.
Edit: the most difficult problem is that capitalism demands perpetual growth, and billionaires and heads of state with a vested interest in growth would never allow the population to shrink without extreme resistance, like pervasive propaganda and outlawing abortion.