[alkonaut]

Yes of course a society in demographic collapse will fundamentally not work. I'm talking about providing for your own generation financially here. Society must manage the task of supplying enough hands to do the work, which could mean e.g. maintaining a reasonably stable fertility rate of at least 1.8 over time for example (Or a corresponding immigration to ensure the same thing). Otherwise yes of course eventually you have a country that is a retirement home with no staff...

[livrem]

Lower population works if we can build better machines. In the best case it can work even for a retirement home country. It requires some political improvements to make sure the machines are employed in a way to make that possible of course.

[alkonaut]

So long as the demographics shifts are gentle enough it's also not a huge problem to have an aging or shrinking population. Higher efficiency will help maintain standard of living with an aging or shrinking population though. The only think important to not have an "end of retirement as we know it" is to not have wars, hyperinflatoin, or a complete demographic collapse (e.g. extremely low fertility rates for a generation).