How much of this can be explained by exchange rates, i.e. the falling US dollar vs other major world currencies in the last 10-15 years? Those plots are all in US dollars, so I would expect if all else is equal that non-US salaries would go up just from that.

I'm not an expert on this stuff, is the falling exchange rate in any way a symptom of the underlying issue, a cause, both, neither? Everytime I try to reason through exchange rates and the world economy I get a headache.