There is really no problem in putting some of the money od SS in the market, provided that no one is "allowed" to try and do timing, namely change from one class (bonds or stocks) to another class, since 90% will do it wrong (we know that is a fact since "smart people" as well as the public always get extremely bearish at bottoms and extremely bullish at tops). If it was set up that every month (it would be best if the investment would be distributed over the whole month as are disbursements of SS payments), the set aside money buys a given amount of SPY or DIA or even QQQ and that is not changed till retirement, that could provide over the long run good results, even better than inflation protected treasuries. What I don't understand is why can't the SS trust manager do that for everyone, why add another 1% cost to those funds through "managers". That is simply a "gift" to wall street, which has not yet proved it is honest enough to be left with that management task.
The real solution would be to take all the inflow of monies, split in four, 1/4 in TIPS, 1/4, in DIA, 1/4 in SPY and 1/4 in QQQ.
If we let individuals manage entries and exits from one class to another, we end up with 90% of our population not having sufficient SS funds by the time they retire. I think that enough people have lost their 401K and IRA accounts in the last 15 years to prove that management of money smartly, not only takes a lot of time, but is also a question of training most people do not have and will probably never have.