[lawn]

Investing everything in a single country, even if in the index, has never been a good idea.

The advice is to invest as broadly and globally as possible, which has resulted in very good return during the last 35 years.

Also, nothing "always goes up". Index investing over long periods of time simple gives you the best risk adjusted returns.

[infecto]

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.

[mekster]

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.