Sigh.
Do those funds deliver those returns EVERY SINGLE YEAR?
Do these funds GUARANTEE in an enforceable way, those returns for any given five year period?
No. They don't. They can't. And if you force people to invest a portion of their retirement savings (SS or otherwise) in such funds then yes, for may it will work out. Most, probably. and the earlier the better.
But it won't work for everyone. And if its a big enough downturn, for long enough, and at the right time, for many people it could be a disaster. Do we back those people up, like we did the banks?
your problem is that you don't understand how bad of an "investment" SS already is.
if our country goes to that much crap where we are -15% for 3-5 years each year, we have a lot more problems than worrying about what the retirees do. and we as a country aren't going to be in any type of shape to help anyone. the only time we have something like that was the great depression, but that was after 4 years with 120% annual growth *in some markets*, with unregulated growth.
there is pretty much zero way, as in has literally never happened in this countries financial investment history, you would end up worse off than SS. for anyone who makes over a pretty low amount of money SS is already a NEGATIVE ROI. so yes, I would consider that a guarantee.
IIRC the breakdown is: if you make less than 40k a year, SS currently gives you more than you put in. but not because of an ROI, simply because it takes from others to give to the poor. If you make between 40-60k a year, SS is breakeven. if you make over 60k when you retire, its a negative investment. under any type of reduction of SS benefits, and those break even points shift much much lower.
under a privately held government mandated investment program, every single group above would come out in the "better" category.
and here is the other point you are missing. if you retire at 65 and the markets are crap, unless you are going to die in the 3 year mythical window you created, your money will still be in the market at 68 as things rebound. just minus whatever you pulled out. There would also be nothing stopping you from pulling your money out at 65 and stuffing your mattress, so that you don't risk losing anything to market changes. a privately held retirement fund would offer flexibility that SS will never cover.