KDE has been around a lot longer than Ubuntu and will continue to exist for a lot longer. You mention the word "spiral" but I would say KDE has a lot more legs than you give it credit for.
I wonder if this is not a positive thing after all, for a mature project. I do not think KDE should add new features at 200 mph speed. I think Ubuntu is actually going too fast.
I think a fewer users -> fewer contributors -> slower feature velocity -> fewer users death spiral is possible.