I disagree. When it comes to long range planning for retirement, the KNOWLEDGE required to successfully invest your savings, the DISCIPLINE required to create an adequate retirement, and the FUNDS available to the individual in order to actually contribute to this plan, a significant percentage of the population is NOT "adult" enough to succeed. What a plan like this would do is to create a brand new indigent class of Seniors. And these indigents would then fall to the government to rescue.
The stock market and other investment avenues are much too volatile and complex for the average waitress, or construction worker, or truck driver to maneuver successfully. And these folks don't have the extra cash to squirrel away in the first place. Most of them are just getting by from paycheck to paycheck.
I used to believe that having the freedom to dictate how Social Security finds would be invested was a great idea. And for me and many others, it might well be. But for what I would guess is a MAJORITY of Americans, it would be a catastrophe. I have changed my opinions considerably over time.