No need to change the identity of having many options and customization in KDE. This is why we have a choice of multiple desktop environments (and window managers too). If there was no XFCE or Gnome as an alternative, or even the upcoming COSMIC desktop, then I would agree to slim down KDE.
I used Gnome 2, Unity and Gnome 3 all for multiple years and have experience in XFCE as well. Really KDE is not much more buggy than Gnome 3 in example. In fact, I had lot of problems in Gnome 3 such as always breaking extensions and other limitations as well, why I switched to KDE in the first place. I was about to go back to tiling window managers, but KDE works good. I encountered with every desktop environment and window manager problems like these. So to me KDE is not really worse.
I really think Cosmic is the ideal desktop, at least from the idea what they want to do. But I don’t want rely on it as its not proven yet (I mean with first release). Maybe in a few years from now.
I have my gripes with Gnome, so won’t go into it now. Not very healthy doing that. :D I do think if you use Gnome as intended and without (or almost none) extensions, and use Gnome Apps for the most part, and don’t need to customize each and every corner, AND embrace the Gnome way of doing things, then I agree it is probably the best DE. But these are lot of ifs and buts. I don’t know if most people fall into this category, I certainly don’t.
And on top of it is how Gnome… well I said I won’t go into it now. :D I’m sure you are familiar with all of this.