This is kind of interesting. Michael Mace discusses successful mobile platforms, but seems to stumble on what a successful platform really is. Was Palm successful? Yes, it was, for a certain period of time. The same goes for all other platforms. None is going to be around forever. Not even Symbian, Windows or whatever.
That doesn't mean that you can't develop apps for a certain platform. It might be around for 10 years, or 5. Or whatever. If you want to look cool in front of your friends, prepare for change, because it's going to happen sooner or later, and then someone is going to point at you and say "your platform failed". Yeah, maybe the platform failed, but that doesn't mean that the stuff you produced for it did.