The boglehead philosophy involves slow accumulation of an index portfolio over time. Not many people had a huge windfall in 1989 and put it all in the Nikkei at once. The money you invested in 1980 is up 500% before dividends.
You are not entirely wrong. The thing is though, the US has been an economic powerhouse on the global stage for decades. The US health impacts much of the rest of the world's markets.
While that is true, there's also no guarantee that we do not see a stagnation of the world stock markets for decades. World indices are also highly concentrated to a few countries, e.g. US is more than 60%.