Even though AVIF decoding support is fairly widespread by now, it is still not ubiquitous like JPEG/PNG/GIF. So typically services will store or generate the same image in multiple formats including AVIF for bandwidth optimization and JPEG for universal client support. Browser headers help to determine compatibility, but it's still fairly complicated to implement, and users also end up having to deal with different platforms supporting different formats when they are served WebP or AVIF and want to reupload an image somewhere else that does not like those formats. As far as I can tell, JXL solves that issue for most websites since it is backwards-compatible and can be decoded into JPEG when a client does not support JXL. I would happily give up a few percent in compression efficiency to get back to a single all-purpose lossy image format.