@hrw The old JPEG is exceptionally good, like alien technology from the future.
Also interoperability is more important than anything else. People care more that the files will work everywhere than about file size savings (GIF is an extreme example of this).
JPEG XL is a very complex format, because it's trying to beat every other format at everything.
@kornel I assume you've heard the rumours about Apple making JPEG XL the new default on its phones?