That's just not true. Most of KDE's 3->4 rewrite was about rewriting from Qt3 to Qt4 for better API and also introducing abstraction layers like Solid and Phonon only and only for the sake of API stability.
If the APIs were stable, there would not be a need to drop one for the other.
Look at the kernel, there have never been a situation where one have to go "sorry, but i only support X+1 from now on".
Similarly, until the 64-bit version, and that in turn was because of hardware not software, one could run binaries from Windows 3.0 (at least) on present day Windows.
That is the kind of stability i am talking about. That is the kind of stability that get third parties to stay with a platform.
Fluxbox was easy to configure with multiple monitors and customize in general. Just tried xmonad with xmonad --replace, and crashed X. Which has been my only experience with xmonad honestly.
Edit: I do like gtile and some other gnome extensions.
I run xmonad at the moment. The learning curve is huge if you don't know Haskell. It took me ages to get it set up and working. But after I learned Haskell, it's really very nice.
Will need to work on getting these features through xmonad, or something else. gtile itself seems to be js.