> Hopefully my photo processor will accept JPEG XL in the near future!

Aren't print shops, machining shops, other small manufacturers etc. ones that always lag behind with emerging technologies?

Designers might also be hesitant to use an untested file format for print, too.

If there’s a large amount of paper that’s been purchased for a job, I definitely wouldn’t want to be the one who’s responsible for using JPEG XL and – for whatever reason – something going wrong.

Pixels are cheaper than paper or other physical media :)

Yes, because those systems cost gobs of money. You don't replace them just for the hot new thing.