> If only desktop environments could do the same!
But they do! Both KDE and GNOME support co-existing major (eg ABI incompatible) versions of their respective libraries.
At least KDE even supports parallel installation and execution of multiple versions of the actual desktop and individual applications. Of course integration is not perfect, but that is to be expected. Combining pieces of two different major versions of a DE works about as well as combing pieces of two different DEs, quite possible, but not trivial to configure.