End-users don't want to have to learn an entirely new UI (read, a different way of doing things; or, "Where's my Start button? Everything I know how to do is under that.") every couple of years. Not because they're (all) dumb, stupid, or lazy.
It's because end-users view a computer as a tool to do what they need/want to do--quickly and efficiently. Anything that distracts from that (like having to re-learn where everything is, and how to do the task they've done the same way for several years) is a negative and annoyance.
Unfortunately, the technology community has forgotten that.