I think their analogy is that your investment in the Prussian/Holy Roman Empire/Carthage stock market can still end up ruined regardless of time.

And yes, that's true. Geographic concentration is a real risk.

if you purchased a world wide index, you will not suffer from such risk.

World indexes are 50% American stocks, they are not as diversified as you think. They’re also vulnerable to mass panic during a crash.

50% American sounds good to me though? And of course a global financial crisis is expected to hit global stocks. That's completely fine.

Yeah, that definitely is the best way barring the existence of aliens. :)

Dinosaurs beg to differ