I don't want to interrupt this nice tribal "capitalism vs unicorns" rant, but...
Decline in demographic growth is primarily a threat for social pension systems, where generation n pays pension for generation n-1 from their employment taxes.
If you had 10 employee contributing to 1 person's pensions say 50 years ago, and in 50 years it's going to be 1 employee, you can imagine it won't work.
My point is that the definition of "capitalism" is so broad that any system that builds its the negation of capitalism is ultimately to end up as a parody of its own ideals. You can't build something just on being anti- something else. And this is precisely the main point of the "capitalist society" memes.