[ketralnis]

> The question is whether the power and influence of the U.S. will grow similarly over the next 150 years as it has over the last 150.

I don't think that's required. Most of these analyses use US stock data because it's so easy to gather compared to international data. The do trends hold internationally, but the magnitudes are reduced. So if you think the US will regress closer to the international mean (and I'd agree) then you can use things like the shape of the bell curve, just not the height. And indeed, this bears out if you look at the markets of the UK or most of the EU. Pretty much any reputable adviser will tell you that that's the consensus, that future returns will probably be lower for the next few decades than they were for the last few. (Usually you see this in the media amplified to a more ridiculous version but that's modern clickbait reporting for you.)

There are other possibilities like we could stagnate for 3 decades like Japan. But yes, that's investing, that's the nature of the bets you're taking.

I'm having trouble finding the quotes but around the turn of last century British economists were looking at the US's explosive economic growth compared to the UK and attributed it to the US having the equivalent of a sudden injection of capital in the form of a whole continent full of free real estate. That is, they reasoned that the UK's growth was limited to what they could do on their existing, mostly already owned and developed land but the US had more physical space for the balloon to expand into. They reasoned that soon that would happen though and the US would grow to fill that space and eventually its economic growth would slow down closer to the UK's. That clearly didn't happen then. I don't think the lesson is the US is exceptional and will continue to outpace the world forever, but I do think that a lesson is that predicting this stuff is hard and reasonable-sounding ad hoc hypothesis don't always bear out.