Indeed, historically stocks have offered the highest possible returns of all the asset classes. Ibbotson Associates, the Chicago-based consulting company, provides some statistics that demonstrate the high performance of stocks. Since 1926, the stocks that make up the S&P 500 (a listing of 500 commonly traded large cap stocks, including such titans as Monsanto, Microsoft, Campbell Soup and General Electric) have achieved an average annual growth rate of almost 12%. That's nearly double the rate for the next most historically lucrative investment choice, long-term corporate bonds, which have grown at about 6.5%.
http://www.infoplease.com/ipa/A0872962.html
So your assumption that mutual funds won't achieve those type of returns are flawed. Considering that we have had a World War, numerous natural disasters, terror attacks, anything else you can think of and still the 12% return on mutual funds exists, that's over a hundred years of past performance.
No, retirees money should have the option either mutual funds or bonds. Not government ran, low return, retirement would definetly be better than the current system. FYI, I'm not for government ran retirement to begin with, but I do acknowledge the catch-22, you have millions of seniors who have paid into the system.
I think you if you take billions of dollars and invest that money in the stock market, it would definetly be a huge boost. Where does your assumption that it would create rising prices/low yield come from, your opinion? Don't you think by growing stock prices that would also increase the dividends that stock owners would get? It's a win-win situation, decrease the overhead for SS as well increasing returns on people's SS payments.