Earlier quoted context omitted.
The ultimate point of these discussions is coming to a conclusion about "what should we do?". I can give you quite a few reasons why dumping all your money in market index funds could end in disaster. I'm not under the illusion these gains are guaranteed. But what the hell else am I going to do?
Invest globally instead of throwing it all in the S&P500, and add some other asset classes.
I don't think our collective future as a planet is any better than the US outlook, personally. If anything, the United States is probably better situated to win future dystopian contests than most, too.