That may hurt JPEG XL adoption, which is a shame, because it's a much better format.
Probably not. One of JPEG XL's features is the ability to losslessly transcode existing JPEG images to JPEG XL while also reducing the file size:
Reducing the size of existing image collections with zero quality loss will make JPEG XL a success no matter what else happens.
https://github.com/google/brunsli
I personally use the cbrunsli/dbrunsli cmdline programs to archive old high-resolution JPEG photos that I've taken over the years. Having a gander at one subdirectory with 94 photos @ 354 MB in size, running cbrunsli on them brings the size down to 282 MB, which brings in savings of about 20%. And if I ever wanted to convert them back to JPEG, each file would be bit-identical to the originals.
Perhaps it's a little early to trust my data to JPEG XL/Brunsli, but I've ran tests comparing hundreds of MD5 checksums of JPEG files losslessly recreated by Brunsli, and have not yet ran into a single mismatch.
I can only say that I am very excited for the day that JPEG XL truly hits primetime.