JPEG XL has a party trick where you can losslessly transcode JPEG files to JPEG XL for a ~20% size savings. I imagine that every CDN out there will enable transparent conversion of JPEG to JPEG XL and that will drive adoption just by itself.
I wouldn't be too surprised if every big CDN already does something like that internally as a competitive advantage, and if so then the incentives switch around and there's no benefit publically switching to JPEG XL - when everyone has it, there's no easy way to do better than everyone else.