This is a great point and one I will cover in more depth in another post. You are correct that the US stock market is an outlier in terms of market success when compared to other countries. This is why I recommend a diverse set of assets which should include a good amount foreign exposure.
As for Japan specifically “Just Keep Buying” would have worked until the late 1980s when they had their bubble. Since then their market has been mostly flat, but a Japanese investor with foreign exposure would have fared better.