If the alternative was putting the information in some hypothetical file attribute with similar or greater level of support/availability (like for filtering acr

If the alternative was putting the information in some hypothetical file attribute with similar or greater level of support/availability (like for filtering across various search engines and file managers) then I'd agree there's no reason to keep it in the file extension in particular, but I feel the alternative here is just not really having it available in such a way at all (instead just an internal tag particular to the JXL format).