The US retirement system (Social Security) is paid for the same way. The population pyramid for Germany does show a much larger hump of upcoming retirements compared to the US though. The ~30yo population cohort has grown in recent years though (through immigration), so maybe it won't be as bad as you expect.

Why do you think Index Funds would solve the upcoming issue?

https://en.wikipedia.org/wiki/File:Gemany_population_pyramid...