Cash: Gets eaten away by inflation. Although the CPI doesn't indicate high inflation it only measures consumer goods. Inflation is there in the price of investments. If you don't invest now, it'll cost you much more in the future to own assets with positive rates of return.
Bonds: Near 0% interest rate, practically no better than holding cash.
Real Estate: Not nearly as liquid as stocks, but the price of real estate is propped up by similar logic.
International Investments: Now this could be interesting if capital flight from the US begins occurring. However, every other economy is hurting like the US's or has significant problems with transparency and whether investors can get their money back out again.
Stocks are more than just their market price. They represent ownership in a piece of the American economy and its future dividends. As of 2016, the richest 10% of America owns 86% of its stocks / future economic output. With the economy plunging while stock prices remain high, this means the fence between being a renter and a owner just got even higher.