If this is the complaint the dollar is safe. And international holders of dollars aren't idiots. They hold Treasuries, which more or less preserve their purchasing power. (Nobody outside the poor, who have to, and nutters, who don't know better, hold cash as an asset. It's so thoroughly assumed that in finance, cash refers to cash and cash equivalents.)
The constant devaluation makes it unappealing.
> My current hypothesis is that the growth in the _price_ of the US Stock market (eg S&P 500) is actually devaluation of the dollar.
I generally agree. But economic orthodoxy says 'Inflation is only 2%', despite precovid, it seemed more like 4%.