JPEG XL's party trick is that you can losslessly (and reversibly) transcode regular JPEG images to XL images, at less CPU cost than fully decoding and then encoding, for a size savings of ~30%. I think this will be enough to put it over the top and drive adoption as CDNs will be able to transparently serve either JPEG or XL for a resource originally uploaded as a standard JPEG depending on whether the requesting user agent supports XL.