As far as I can tell, this makes the assumption that American stock market returns are normal. I would argue that global stock market returns are far more normal. What have returns on the German stock returns looked like over the last 140 years, just to pick a counter-example?

In the past 140 years, the United states has gone from backwater to superpower, has enjoyed stable & good governance, has not been invaded, has been able to plunder a large amount of "greenfield" resources, et cetera.

The next 140 years cannot be as (relatively) fruitful for the United States. When you're number 1, relative growth is hard/impossible, and you must rely only on absolute growth.

The trivial answer to the question in the title is "you cannot save enough to account for everything". If you're saving in dollars, you can be wiped out by hyperinflation. If your investments are on the NYSE, what happens if it gets nationalized after a revolution?

Of course, the answer is that you pick a level of risk that you're comfortable with. Anybody that tells you anything is 100% safe is misleading you.

And remember that rare events are much less rare than most models predict (cf Taleb).