If you just look at the past 20 years, the US has had exceptional returns compared to the rest of the world.
The thing is, historically, high PE ratios like what we're seeing in the US do not correlate with short term returns that are as high. Expected future returns decrease as the PE ratios go up in a pretty linear fashion.
https://am.jpmorgan.com/us/en/asset-management/institutional...
If you want a different point to backtest from, try Japan in the 80s and early 90s