If RIM's strategy doesn't have some component that involves trying to woo iOS developers to their platform, I predict they will fail.
Finally, the whole smartphone area is ten years old, consumer-focused part of it less than five years old: less than mature, and way less than settled. In another ten years, this all might have turned out to be a silly bubble as Moore's law erases any effective differences between smartphones. Will Apple "fail" when its percentage profit per device and market share falls to current Macbook values?
I think it's less important how "good" an SDK is unto itself, and more important how good it is relative to it's competition. In this case you have to compare the Playbook SDK to Apple's and Google's because when developers are making decision about which platform to work with, they're going to be choosing between them.