Too risky IMO, when you're gambling with the safety net SS provides.
What happens if the market or investments go to total crap?
You going to tell those that rely on SS that, "oopsie, sorry about that, now you get less"? Not to mention, that right now seems like the absolute worse time in history to try something like you propose with more and more boomers drawing SS.
I've personally been willing to take much more risk with 401's knowing that I have the safety net of SS, rental income, and also a Gov. pension. It's paid off for me in a pretty big way, but even if it didn't, I would still be in much better shape than a vast majority of Americans.
Knowing that lots of Americans would be totally hosed without SS, I would not be comfortable with gambling their futures in the market.