Capitalist society: Don't have a baby unless you're financially able.
Also capitalist society: You want a liveable income so you can afford kids? Lol, no.
Capitalist society, yet again: Why aren't you having babies? We need you to produce the next generation of labour, please.
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.
@kravietz Any alternative is a unicorn, I take it?
Anyway, sure, that's also a threat. I'm not exactly sure what your point is, though, in the context of what I've said.
"Capitalism" is extremely broad, non-prescriptive and non-binary system. If you have prices and free market, you essentially got capitalism.
Marxism on the other hand is a closed and very prescriptive ideology. When Lenin introduced (pragmatically) NEP in 20's, he was heavily criticised for non-compliance with the Scripture. Stalin fixed this :)
@kravietz "If you have prices and free market, you essentially got capitalism." Yes, but also know. Free market anti-capitalism (mutualism) is a thing, albeit kind of a niche ideology.