But GUI toolkits work very different; ProgA can link against GTK, ProgB against Qt, ProgC against Tk, and that all just peacefully coexist without problems (sho

But GUI toolkits work very different; ProgA can link against GTK, ProgB against Qt, ProgC against Tk, and that all just peacefully coexist without problems (short of perhaps some UI inconsistencies). None of these toolkits have to even be aware that the others exist for it all to work; there is no shared protocol (other than the system itself: X11, whatever Windows has, etc. – and look at how long s/X11/Wayland/ is taking, and there is considerable disagreement whether Wayland is even an improvement in the first place). But in a terminal all code runs in that terminal, and terminal programs don't "link against" a terminal in the same way that you do with a GUI toolkit. You need to implement a shared protocol on both ends.