You have a large amount of workers who are paid to invest. They can't put it in the bank, and count on the interest. In order to keep their job they need to invest. They will invest until it crashes? (When it does crash, I hope the haves will take pity on the have-nots; just a little, and not just to the obvious ones.)
Not to detract from the general point, but during the last bubble crash, we ended up being forced to invest via bailouts.