There are numerous problems with partial privatization. Shifting Social Security taxes into individual investment accounts would mean a big pay day for Wall Street and significantly higher administrative costs. Bush's plan is risky for those who count on Social Security for an important part of their retirement income. As recently as the 1970s, the stock market was flat for a decade. These are important concerns, but the problems with Bush's proposal are much more serious than unrealistic assumptions, unpredictable returns, or a bonanza for Wall Street.
Partial privatization threatens the retirement system itself.
In recent years, the program has built up a trust fund, but it still relies on taxes from current workers to pay retirees' benefits. Bush claims he can divert part of current income to private accounts and still maintain benefits for current retirees without raising taxes. That's a pipe dream. If you cut income and maintain expenses, something has to give. That's much worse than the 37-year timetable proposed by the most pessimistic critics of the current system.