The problem isn't localized to first world countries anymore, the process in all countries has already begun and cannot be stopped. Almost every country on earth has a replacement level or below fertility rate and every single country has a fertility rate significantly lower than 30 years ago. A generation has already grown with the lower fertility rate, which means a generation is already locked in that will age out of the workforce with barely enough young people to replace their productivity. It's done.

There might not be a sustainable solution, besides a brave new world, but I personally would like to preserve human dignity. It appears that steady state population sizes are a rarity in the natural world and that almost all populations are chaotic and fluctuate. https://fractalfoundation.org/OFC/OFC-6-1.html