Let's say there are areas to improve still, but introducing anything new requires investment of time to learn it, breaking habits and giving discomfort. Even if it eventually starts paying off, it can cause a push back naturally. So it's really hard to do it right, may be only very gradually? Surely not like Gnome tried to do it.
Some direct comparison with such idea could be let's say a suboptimal keyboard layout and more optimal ones that are barely used.
I personally stick with KDE exactly because "it just works well enough" and its flexibility at the same time.