As a reference point, I’ve stubbornly been using A4 paper in both USA and Canada for years now with governments and businesses. Never had any problem with IRS, passport agency, CRA refusing to accept them, or even mentioning it at all in person.

No problem buying it either, though it’s about 2x the price of letter. (I go through about 100-200 sheets per year mostly due to my daughter drawing on it.)

I don’t actually store any printed paper since anything gets scanned and shredded, so no comment on long term filing.

(Due to this experience I recently partially switched to B5 paper, which is a nice size compromise instead of A5 which would be too small. Anything A4 will print to it without scaling issues of course, just takes up a bit less space.)

I think it would be pretty rare to find a paper "storage or processing " device that can accommodate letter but not A4.

The US would need a president who is bullheaded enough to executive order a thing, like drop the penny or paper straws, but that also likes Europeans, so we might be waiting a while for a push from the US government. Printed media nerds, however, already know about benefits of A* and will switch as soon as they're in power and can spend the millions and millions of dollars it would cost their company to switch. Printed media is not as important as it once was, though, so the switch is really to start outputting PDFs as A4 and letting people print scale-to-fit when they actually need to print something you've sent them. Given how often my printer sees use, I doubt most people would notice if the PDFs you send them started being A4 instead of US letter.