I think an easy answer to this question is “features”.
XFCE and KDE are on near opposite ends of the spectrum when it comes to feature development. Heck, even Wayland isn’t really working on XFCE. There is no way you will be able to achieve feature parity by hacking together some third-party extensions or whatever.
It could be argued that what makes XFCE stable is the lack of fancy new features. If you want to add a bunch of extra software and features to any DE you will invariably add additional bugs and instability as well. As they say, you can’t have your cake and eat it too. 