Dalio's long-term views still seem to revolve around his big cycle theory but more recently he's admitted that China has some big problems to deal with, including the demographics issue: https://www.linkedin.com/pulse/china-100-year-storm-horizon-...
He's not the first guy to talk about this long cycle, great powers wax and wane. He's also right that the US is looking weak by certain metrics, chiefly at the rate US debt is accumulating you have to wonder how much longer the dollar can remain the world's reserve currency. But what he didn't talk about in the book and is now being forced to admit is that China's problems are at least as big as America's (if you believe that demography is destiny, China is going to lose in the long game for sure). It's entirely possible for countries to challenge even a weakened hegemon and lose, happens all the time, we may be seeing it happen now.