The UK did an experiment that lasted some 25 years starting in the 1980s where anyone could “opt out” of the UK version of SS and invest half of the payroll taxes they and their employer were paying towards SS. My BIL was one of the ones who chose that route. The vast majority failed to invest well enough to do as well SS would have done, even though only government approved funds could be used. It now means the UK SS has its own version of WEP when folks start drawing it.