The reason that Nokia have to keep moving through OSes is that the hardware for these devices is moving forwards at such a rate. It is not like the desktop market where a 3 year old machine is still pretty capable. A three year old smart phone is like a chocolate teapot (cue barrage of abuse from N770 owners).

You can't keep the OS fresh and taking advantage of new hardware while keeping backwards compatibility. And if you did you'd only end up in a mess as apps would have to be compiled for different architectures and specifications.