No matter the system you'd end up with the working age population tending for the children and elderly. I don't think that in it self is a problem. You could argue around implementation details though ...
(Near-)retirees are typically the largest voting bloc and usually the only one experiencing any growth in most of the West. It is unsurprising that they get to set policies that solely benefit them at the expense of everyone else, including those that are yet to be born and those who will never be born because of those policies. And why would they care about the future? Retirees, by revealed preference, don't, and why would they? It's not their future they're destroying. Indeed, they're making quite a nice (short) future for themselves by bleeding societies dry.