JPEG XL can losslessy transcode JPEG into a smaller format. JPEG2000 (or WebP or anything but Lepton[0]) didn't offer that. Besides, we have gif and png for approximately the same space. gif still isn't gone. Displacement isn't necessary for a new format to become useful.

[0] https://github.com/dropbox/lepton

Even if JPEG-XL could be considered only 'marginally' better than JPEG, and as good or even worse than avif/webP in some specific context, it is unique in the fact that it can also do lossless, HDR, extremely high res, complex color channels, generation loss protection, multi layers, advanced authoring features, etc... It's not a format only meant for the web but plenty of other use cases such as in science, medecine, art print, etc.

And not only that, it's reasonably fast to encode on consumer hardware.

JPEG XL has the ambition to supplant all the images formats of the next 20+years.

> JPEG XL is only marginally better

You will have to define what is marginally better. WebP is definitely marginally better than JPEG. And JPEG XL is easily 30- 40% BD-Rate at the same quality at BPP 0.8 or over.