I should amend my point to say that stock prices cannot acceptably fall without massive intervention by the government. The system is set up so that those who have the most invested in the market (mostly the older generation who are trying to retire, institutional investors, high net worth individuals) will do anything to prop those prices up, and the government will listen because it's beholden to the interests of the older and the wealthy. They'll bankrupt this nation's economy for those that will inherit it to save those retirement funds at the benefit of wealthy investors.
The last month of cliff-fall would suggest your premise is wrong: prices absolutely can go down.