You get better compression and services can deliver multiple resolutions/qualities from the same stored image (reducing storage or compute costs), all transparent to the user.
So your average user will not care but your cloud and web service companies will. They are going to want to adopt this tech once there's widespread support so they can reduce operating costs.
Second Life went with JPEG2000 for textures, and when they open sourced the client, they had to switch to an open source library that was dog slow. Going into a new area pretty much froze the client for several minutes until the textures finally got decoded.
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.
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.
(Having said that I do wish for JPEG XL to become a true successor)
That’s not true. JPEG 200 had substantially smaller file sizes and better progressive decoding – something like responsive images could have just been an attribute telling your browser how many bytes to request for a given resolution. It also had numerous technical benefits for certain types of images - one codec could handle bitonal images more efficiently than GIF, lossless compressed better than TIFF, effortlessly handle colorspaces and bit depths we’re just starting to use on the web, etc.
What doomed it was clumsy attempts to extract as much license revenue as possible. The companies behind it assumed adoption was inevitable so everything was expensive - pay thousands for the spec, commercial codecs charged pretty high rates, etc. and everyone was so busy pfaffing around with that that they forgot to work on things like interoperability or performance until the 2010s. Faced with paying money to deal with that, most people didn’t and the market moved on with only a few exceptions like certain medical imaging or archival image applications. In the early 2000s the cost of storage and disk/network bandwidth meant you could maybe try to see the numbers as plausibly break-even but over time that faded while the hassle of dealing with the format did not.
Amazon alone sold over 375 million items this last Prime Day. Let’s say that was 200 million items loaded/day (ignoring the unpublished number of failed sales), with 9 images (the maximum from a cursory glance) at 2000x2000 for a 1:1 ratio and zoomability. For a 90% quality JPG at 24-bit color that’s 410KB. ((410KBx9)200,000,000) = 738TB. Now imagine cutting that in half with no perceptive difference except faster loading to the end-user.
For end users the other options may be more desirable, but I would argue the importance is in the compression itself.