Presumably, JPEG-XL decoders would produce higher quality decodes out of the same file while being backwards compatible with browsers that only have support for standard JPEGs
If it's backwards-compatible with standard JPEG decompressors, then you've just made a better JPEG encoder. And that's great, and people are still doing that all the time, but we don't need a whole new ISO spec to do it. All of the JPEG-XL enhancements are new additions to the format that aren't backwards-compatible.