For a very short term investment strategy, sure. But if you want to save for 20+ years this isn't really the way to go.
By investing in more comprehensive index funds (not individual stocks - this is almost always stupid) you are basically putting your money into the economy. Don't trust the USA? You can diversify further with a world index fund.
If over the long run the stock markets were to continuously decrease or remain stagnant we'd be in economic turmoil. There would likely be a complete lack of significant innovation and purchasing powers would rapidly decline. Sure it could happen, but that is a total worldwide doomsday scenario.
Keeping your money in cash is far more likely to fuck you over the long run. You lose out on compounding and your purchasing power goes to shit.
If you do happen to spot good opportunities to buy assets (real estate, a business, whatever) you can always sell some of your interest in the market and take advantage of them.