"JPEG XL lets you recompress an existing lossy JPEG losslessly” This is such a niche usecase. Who cares about bit more lossy compression for an already lossy compressed file.
There’s also no reason one should tolerate further loss just because the image is already lossy. Whether the original is lossy or not, you either want to keep it exactly the same, or you're okay losing quality to save some space. Archiving and preservation come to mind as the first reason someone would want to go with the former.
Perhaps for those unaware: "without quality loss". I guess the people who care are those who store a lot of images in jpeg format (at least relative to the storage space they have). In my example, I have >1TiB of photos in JPG format (yes, I know), and it would be nice to get 200GiB back for free without losing quality. For a company offering an image storage product, this would be great cost reductions.