Yes, hypothetically you can have a society in which people marry, have kids, raise the kids, and then split. But that's not what's happening across the Western democracies.

Look, this issue of social reproduction is the biggest threat to liberal democracy, and it goes a long way back. In France, the same areas in which the most copies of Diderot's Encyclopédie were bought were the ones that started seeing both marriage and fertility declines. As the society as a whole proceeded to liberalize you started seeing population declines across the board.

I would add that things like child support benefits and other forms of social insurance -- the routine answer to all social ills in the West -- have not shown any efficacy in addressing this. It's not a welfare problem. Hungary has tried to give lots of money to increase fertility. Romania tried authoritarian techniques to increase fertility (fertility was a chronic problem in Eastern-bloc nations). None of that worked.

If you really believe that modern liberal societies are a viable concern, then you have to solve the problem of more and more people dying alone, having never married, and having no children. You have to solve the problem of people living isolated lives and not being integrated into tight family networks. Because over the long run, these defects will prove fatal.