My preference would be to cleanly separate the API levels such that Android "compliance" requires the ability to update core components. But that's a software engineering problem, because right now the frameworks are evolving alongside driver and middleware stacks such that you can't just drop an "ICS" OS stack on top of a "Gingerbread" kernel and drivers and expect it to work.

Apple's answer, of course, is "... or else you won't get the next iPhone."