I'm under the impression that one of the main selling points of JPEG XL is that it's backwards-compatible, so once everyone has a decoder implementation, every existing JPEG comes along for free without anyone having to reencode any old photos. I suspect consumers wouldn't notice the difference if devices started emitting them, and in the meantime that means there's no harm in continuing to emit classic JPEG for as long as you want.

Tell that to all the users with iPhones chucking out HEIC/HEIF images by default, which are much less convenient

JPEG isn’t good enough which is why we ended up with HEIF, which is a massive pain because it’s patent encumbered so it will never be widespread adopted.

The most glaring issue with jpeg is the lack of HDR support.

Chrome doesn't use hardware acceleration for decoding any image codecs-- not for JPEG or WebP or AVIF. It has some code for it (using VA-API), but it's disabled by default. Hardware video decoders are generally hard to use for image decoding tasks.

Safari appears to be the only shipping browser using hardware decoding for JPEG and HEIF (HEVC based images). This is probably because vertical integration lets them make the decoders appropriate for image use.

Also, "AVIF video" is incorrect. AVIF is the image codec based on AV1 intra frames.