I don't think it's valid criticism in the first place. The kernel is only a single part of a much bigger "Linux desktop distribution". For desktop users, work

I don't think it's valid criticism in the first place. The kernel is only a single part of a much bigger "Linux desktop distribution". For desktop users, work on good default configurations for all software, X (or its replacements), window managers, package managers, backports of security fixes to old software, and many other things that Canonical does are just as valuable as improving the kernel itself.