As it was "impossible" for Apple to decide when all they had was the old Macintosh toolbox and NextStep UIs, but they did it. They made Carbon/Cocoa to look reasonably consistent despite huge internal differences.

Linux can do the same with Qt/GTK+/etc. It certainly possible to have high-level UI consistency between multiple toolkits, especially when they all share the same lower-level stuff (X11/Cairo).