I think you're not understanding my question. You said that the most exciting feature of JPEG XL is the ability to losslessly encode old JPEGs. My point is that, if this is the most exciting feature, then it's unsurprising that the industry is choosing to ignore JPEG XL as the "most exciting feature" is more or less useless.

Technology is a means to an end. Transcoding images is a means. The end is what you do with it. In the 90s and early 2000s, encoding images in JPEG allowed for digital cameras to store vastly more images on their limited storage space. Bandwidth was extremely limited, so JPEG encoding allowed for detailed images to be shared over the web, which wouldn't have been possible in a different format. Today, however, storage and bandwidth are cheap. JPEG already does a decent enough job for old images. Making old images even smaller isn't really enabling any new "ends."

I guess the ultimate question I'm asking is "Why do you want your old JPEGs to be smaller?" What is the end you hope to achieve with smaller JPEGs?