I don't think it's specific to capitalism. Any system needs workers to produce enough for retirees and children. If you have more retirees and less workers any system is going to struggle.

Growing, biological organisms need growth. Because once they stop growing, now they're in dying mode. It won't happen instantly, of course, but they're going to die. And it's this way with civilizations too. Rather than being one of the "disadvantages of capitalism", it may actually be a principle of life itself.