Why do you assume that the benefits would outweigh said costs? That's a weird burden to set on the format. Using JavaScript on the browser to decode it is a huge hurdle, I don't know of any format that ever got popular or got its initial usage from a similar approach. Avif was just added too, even if no one was using a js library to decode it beforehand

Fwiw I agree that there's a weird narrative around jpegxl, at the end of the day it's just a format, and I think it's not very good for lower quality images as proven by the linked article in the OP. Avif looks better in that regard.

I think it would've made more sense than WebP though (which also doesn't look good at all when not lossless), but that was like a decade ago and that ship has sailed. So avif fills a niche that WebP sucks at, while jpegxl doesn't really do that. That alone is reason enough to not bother with including it.