Your point is valid - we shouldn't take single-country risk in investing. Assuming you believe the world as a whole will get more productive and value creating, globally diversifying your stocks is the answer.
As an example that supports your point, the Japan stock market (Nikkei) peaked in 1989 and STILL has not returned to that high.
As an example that supports your point, the Japan stock market (Nikkei) peaked in 1989 and STILL has not returned to that high.
However, even if you were incredible unlucky and had bought in at the 1989 peak in Japan, if you had an internationally diversified portfolio, you would be OK. E.g. a 30/30/20/20 Jp Stocks/Intl Stocks/Jp Bonds/Intl Bonds portfolio purchased in 1989 at the Nikkei peak would have more than doubled by 2014 (see here: https://www.bogleheads.org/forum/viewtopic.php?t=265807 and also https://www.afrugaldoctor.com/home/japans-lost-decades-30-ye...).