It is a problem because that "quality of life" depends on demographics. If there's a huge amount of old people relative to the amount of young people, pension systems turn into a Ponzi scheme. People that are paying into the system now will not be getting their money back, unless the next generation pays even more. Rinse and repeat.

And before you say "we'll just take it from the rich", please do the math on how much money the rich have, versus the cost of pensions. It's sobering.