I keep asking this question elsewhere but none of the leftists will give a definitive answer

The main subject of the OP is declining birthrates and how that is bad for the economy. The leftists keep clucking about how never ending growth is a failure of capitalism. However, I keep pointing out that things like retirement require additional workers:

If people retire they no longer provide labor, but labor must still be used to support them. Either more laborers have to fill the deficit and/or the remaining laborers have to work harder to cover the shortfall, and/or those not working will have to make do with less or they will have to continue to work their entire lives.

No answer about the solution provided by non-capitalist systems is forthcoming and I appear to be blocked. All I get is a chanting like recitation about how capitalism has failed because it requires never ending growth.