Eventually you have to sell products and services to consumers at a profit. The higher your "P" for a given amount of purchasing power the consumer has the more leverage you have to results on both sides, and this means that when the right person sticks up their hand and calls bull**** you're going to see 50% declines or more in price before you can reach for the mouse.
Count on it
Just because I think that debt levels in virtually every corner of the globe are too high and Price/Earnings levels are elevated doesn't mean that we will have a stock market collapse. Markets are frequently irrational. This bull market might continue for quite a while. This is a chart of the S & P 500 back to 1955: