The forecasts and analyses identifying risks of U.S. stocks over the next few years aren't a good reason to change one's investment portfolio in themselves, but they are a good reminder to make sure each person is taking on risk that they are comfortable with.
We know that non-U.S. stocks are highly correlated with U.S. stocks; portfoliovisualizer shows .90 as the r2 over the last ~10 years. On the other hand, the U.S. stock market currently has a high exposure to large technology stocks, meaning a hit to their valuations will impact U.S. stocks more than non-U.S. stocks. We know from experience that it is possible for a specific country's or region's stocks, especially if they have different sector exposures, to decline more than others in downturns or just come unmoored for several years. The classic example is Japanese indexes; they overperformed and then took a fall in the 90s, but since they have been fairly correlated with Europe and U.S.
Here's a portfoliovisualizer graph showing U.S. (Port 1), Europe (Port 3), and Pacific (Port 2) stock returns since 1987. Japanese stocks dominate the Pacific equity universe.
Here's the same graph since 1999--highlighting how closely correlated Pacific has been to other equities.
So, as you can see, it's possible for one country/region to come unmoored from its correlation to global equities for a few years.
And, over the last few years, it has been U.S. stocks themselves that have come unmoored, overperforming significantly as you can see from the first graph above. Now, both Japan and Europe have lower P/E ratios. We've got U.S. stocks exposed more technology, facing the possibility of major trade issues, and looking at other potential headwinds.
Does that mean U.S. stocks will inevitably fall? No. But again, it's a good reminder to make sure we're comfortable with single-country risk exposure and with our equity-fixed income allocations. Past experience shows us the party can end. That doesn't mean we need to leave right now, but it's a good idea to make sure we pace ourselves so we can drive home if the party does stop.