This is why the idea of "runtimes" is bad at the core: existing applications do not benefit from new features and fixes in the latest version of the runtime. But fixing this means that said "runtimes" (ie Gtk, et al) need to actually give a damn about backwards compatibility, which is certainly not something anyone on the desktop side of Linux above X11 cares (and they want to break X11 too instead of fixing it because "it is too hard to fix it so let's throw everything away and make something new that for some magical reason wont also become hard to fix in the future").
Why the idea of having a standard set of libraries to handle common GUI tasks that are available "everywhere" (in desktop distros at least), just like the C library and X11 library (so far) is available everywhere and can be relied on in the long term (think decades, not weeks) is something that sounds like science fiction instead of common sense is beyond me.