For the USA to file nonstandard phraseology differences with ICAO would fill a volume as thick as a telephone directory.
It’s unfortunate enough that they persist with feet and statute miles for visibility, pounds and ounces, US quarts and gallons, and inches Hg for altimeter settings, but they reckon they have a right to those things because (to misquote that wonderfully politically incorrect song), “they got the bomb”.

You're right about the length! I looked up the US AIP and the formal list of differences is huge. https://www.faa.gov/air_traffic/publ...ction_1.7.html. And affirm/affirmative does not even get a mention, so who knows how many other differences there are...

There are about 210,000 aircraft in the USA and 97% are GA. Almost all the GA aircraft will have altimeter sub-scales in inches Hg only. Add in about 30,000 Canadian aircraft plus a few hundred (?) more from Mexico and Colombia and you can see why North America will never switch to HPa.

The other units mentioned will also never change - there is too much entrenched infrastructure that uses those units.

You won't see any metric structural hardware on your aircraft either.

Don't forget that China uses meters for altimetry and Russia below flight levels is in meters. Considering a very big percentage of the worlds pilots are from the USA, China and Russia, you can safely say that overall standardization of the majority of pilots on one system is still a while away.

In any case, no one I know seems particularly fussed. Toss a few warbirds, vintage and just plain old aircraft into the mix and to be a competent pilot these days unit conversions of all kinds are just part of the training.

What isn't are the ATC procedures - and if YT videos are any guide, as far as I'm concerned, the USA are welcome to theirs and good luck to them.