First of all my comments are highly subjective don't take them too seriously.
KDE could (in theory) continue using Qt 3. So it's not like its mandatory to instantly switch or even number releases according to new Qt major releases.
While you're right that KDE is more than Plasma or Nepomuk, they are indeed the most influencing parts of the desktop (being visible part of, crashing or painfully slowing the experience). So if you excuse me, KDE comes down to Plasma and Nepomuk in my subjective opinion. And for the rest of the applications I'm afraid I personally consider only the Digikam as important. That's one of the major problems too. It seems KDE as a platform is not very essential for me.
Contributing to Qt is very nice and respectable but I don't think it will directly translate to making a good desktop.
My whole point is free software can not and should not follow a commercial product too closely, unless it's a commercial product of someone too. Sudden changes is what making our ecosystem worse because free software is all about polish and improvement not about rewrites and dumping working code to trash.