Looks like the Nikkei 225 average dividend yield is around 1.4%. If you bought in 1989 and reinvested dividends it looks like you would have briefly broken even in 2015 (ignoring inflation), then hit solid black sometime in 2017. This is completely absurd and shows how huge of a factor timing is in market investments.

Remember, kids, dollar cost averaging is usually the way to go.

While academically you are correct, I am not sure I would say it has never been a good idea. I have a US bias but most portfolios in the US have a significant if not total weighting on domestic markets. Even in my memory of the last 10 years there have been continuous questions on how much international exposure one should have. Most of the time you see something like a 5-10% exposure in international markets which in theory gives you the "best risk adjusted returns" but in reality since the US has been a continuous powerhouse, gives you a lower overall return.

Many people, especially in Japan don't realize not investing and simply saving means they're actually investing in their own country long term, which for Japan, doesn't look like a smart strategy at this point.

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%.

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.

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.