this has nothing to do with backward compatibility, this is a solid case of "I don't want to pay you for features that my customers need, and I can't provide decent replacement, but my users will buy my stuff anyway, so why bother"
No, it has to do with Google putting restrictions on the use of map data, including not allowing turn-by-turn navigation. Apple knew that it couldn't have a core feature of its product permanently beholden to a competitor and the restrictions it might impose down the line. So, realizing that bringing maps in-house would not get any easier down the line, it decided to rip off the band-aid now. Every other smartphone platform has done the same. Microsoft uses its own maps. Nokia owns Navteq and Google we all know about.