What backward compatibility are we talking about here? Backwards compatibility of images will be fine, backwards compatibility of decoders might be impacted, but the article says the major image viewers (browsers) and image editors already support the 3rd version. Better compression is only planned for the 5th version of the spec.

Also if you forbid evolving existing formats, the only alternative to improve is to introduce a new format, and I argue that it would be causing even more fragmentation and be more difficult to adopt to. Look at all the drama surrounding JPEG XL.