But that's kind of the thing with JPEG XL. It's designed specifically so you can store everything as JXL and "downcast" to older formats for little to no cost.
JXL downscaling is practically a free operation that allows you to very effectively deliver multiple image sizes from a single copy.
JXL -> JPEG re-encode is very cheap and results in nicer looking, better compressed JPEGs than using traditional JPEG encoders.
And for formats like PNG, JXL should be able to hold that same data in roughly the same size (or less) without any visible data loss (or none at all for a slightly larger filesize). So converting down to PNGs at different sizes just becomes it's own pipeline if you still need that (over JPEGs or JXLs).
It's the first thing close to a universal image format which is part of the reason some people push so heavily for it.
For huge majority of (even picture heavy) websites you won't get more than 10% of loading time reduction even in the cases where JXL most superior.