[Tyrannosaurs]

If you're a corporate who've invested significantly in training, business specific software, support infrastructure and so on for a particular platform to be told you've got to start over is a massive deal and you care a whole load that your massive investment is largely for nothing.

Backward compatibility would have allowed many corporates (a significant market for RIM) to maintain that investment and if they could do that that would be a major reason to stick with RIM. The fact that RIM have cut them loose effectively means they have to start over and makes RIMs incumbency effectively worthless when it comes to business retention.