Wealth creation requires work. Retirees don't work, but still require a portion of the wealth that workers create to live.
It doesn't matter if it's a government pension or a 401K investment, a declining worker population will mean that you'll have the exact same retirement problem. Government pension require people working and being productive and paying taxes. Your investments will only produce returns if people are working and being productive (in which case they are also probably paying taxes).
Unless, you know, you do some wealth redistribution, away from people who own all the means of production, to the people who don't. Then they former will be the ones with a problem. It won't generate more net wealth, but it would certainly prevent a large portion of it from being locked up in the hands of a small elite.
Or, alternatively, more automation and high taxes on robots.
Privatizing the problem of pensions is just an accounting trick. It doesn't actually solve the problem.