Before the baby boomers we had global war, before that the great depression and growing literal communism, before that prohibition and the mob. And of course prohibition was prompted by social problems around alcohol.
So I don't think "problems compound" is always true. For a long time, teen pregnancy and alcohol use were on the rise, but that trend didn't continue indefinitely, and both have relatively recently had dramatic declines.
If you look closely enough at any time period there are often multiple serious political, economic, and/or social problems, so much so that it seems to be the norm rather than the exception.
> Post-stalinist Soviet Union, while not manically murderous, eventually imploded under the weight of a myriad suboptimal policy choices
Even this I'm not so sure. Here is a graph of their GDP [1]. My understanding is it was a political collapse first (a failed coup), and the economic collapse followed. An example that I've read China has been very mindful of (apparently Xi talks about Russia much more in his books than he does the US).
> I do wonder how US/China rivalry will play out in the 2040s and beyond.
Me too, at that point they'll probably be a much larger economy than the US. Will that have a destabilizing effect (especially with a potential military conflict over Taiwan)? Or will (the US) having a shared rival be somewhat unifying? The UK did OK despite declining in (relative) importance, but I could imagine the US struggling with its domestic problems even without any great power rivalry...
[1] https://en.wikipedia.org/wiki/File:Soviet_Union_GDP_per_capi...