I totally agree with you, but that is often not how reality works. For example Norway when they found oil they put (most of) the income from that in a fund and now only use the interest they earn so it is like an infinite source of income. The Netherlands on the other hand invested their money in infrastructure, social subsidies and other stuff that was useful to the people alive when the money was flowing. Now I don't want to say one of those choices was the better choice, because that is actually not my point. Because either you save up money when the population is growing (meaning people in that time frame have less benefits), or you spend your money and you have a deficit later on. In either case a group 'misses out'. The same here, because the population is getting older and less people remain to pay the bill, the working people have less money left. In a perfect scenario when this money was saved up earlier, that money wouldn't have been spent before. Either way, and I agree the society should take care of it self by organising an economy that benefits the society, you end up in a situation were the expenses have to be paid. The right argument is on whether it is worth spending the money (which I think it is), but the argument made is that it only is a problem for capitalists /in a capitalist society and that is just not true. Also communists or for example hunter gatherer societies would face the problem of increased costs of supporting a part of the population that is to sick and old do work.

Sorry long ramble, but it being true that it's not a problem when you zoom out enough doesn't mean it's true that it's not a problem for the individuals alive during that time frame.