My understanding for the reasoning behind this is that they don't want apps to have to ship the runtime stuff to avoid downloads and app sizes getting large.
That could be fixed by just shipping an updated shared library to all phones similar to how Google play services works on Android, but I guess they figure if you're updating anyway you might as well just update the whole OS.
That could be fixed by just shipping an updated shared library to all phones similar to how Google play services works on Android, but I guess they figure if you're updating anyway you might as well just update the whole OS.