Since the end of the 'Great Recession' in 2009, US stock markets have outperformed those of the rest of the developed world and the emerging markets.
Why? Supply and demand. The number of publicly trading companies in the US has been cut in half in the last 20 odd years. Since the 'peak' in 2007, the average daily volume on the New York Stock Exchange has also been cut in half. At the same time, the number of foreign public companies has more than tripled!
Is the decrease in 'supply' a problem? No, while it can increase volatility in rapid moves up and down as there are not only fewer things to buy, but also fewer to sell, the supply situation is actually very positive given the strength and structure of the US economy, capital markets, and demographics.
It isn't just US investors driving US stock markets higher, investors from all over the world send their money to the US due to the liquidity, general transparency of our companies, and our legal structure. That should not change.
The fluidity, competition, and liquidity of our economy forces change in our corporations to grow and sustain profitability and that has an evolutionary and sometimes revolutionary effect on our 'market structures'. That is a good thing.
With lower supply, increased access to capital, growing wealth and solid demographics (the Millennial Wave), US stocks should be a good bet for the future.
*this is not advice or an endorsement to buy stocks, that is based on personal analysis of your own situation*