It does seem likely that a future edition of PDF will include JPEG XL.
JPEG XL is available (in beta/with a flag) in Chrome, Edge, and Firefox. Seems likely once it's no longer in beta that some web developers will start conditionally using it depending on the user's browser.
It doesn't need to be useful everywhere to be useful.
Not gonna fly, sorry. Come back to this comment in 5 years and you’ll see.