You can't just borrow or create money to fund things that are not profitable. Not saying infinite population growth is desirable but spending the large amount of resources on old folk does mean not spending it on the young folk = less money to education, health care and infrastructure. It's not fair to reduce real world problems to 'you just need to spend your money wiser'
Edit: just to clarify the comment I made above, it doesn't say we shouldn't care for retirerees, it is saying you can't keep the price we pay for supporting them the same if the size of the group if old people rises and the group of people who work to pay for it shrinks. An aging population is a burden to any population in any financial system just like a growing population is a boon. Again, that doesn't mean we shouldn't care for old people.