For the next few months at least, it looks like JPEG XL is just going to be one of those formats for Apple users, like HEIC. With people complaining about programs not supporting WEBP all the time, JPEG XL isn't going to gain much popularity in the mainstream either.
At least people supporting Safari will be able to make use of the format for faster load times soon; the <picture> element makes the transition quite painless after all.
An interesting difference with the WebP situation is that any app that uses Apple's codec framework on iOS/iPadOS, which is almost all apps that deal with images, will support JXL automatically.
Actually, I just downloaded a JXL via Safari, and after confirming that it was still a JXL, I tried using it in a context where JXL isn't supported, and it automatically turned into a JPEG.
At least people supporting Safari will be able to make use of the format for faster load times soon; the <picture> element makes the transition quite painless after all.