>Second, they want a familiar interface. Third, they want off-the-shelf software (and whatever legacy as well) to run.
Those points lead into a situation where many people only upgrade their software when it's crammed down their throats by a hardware upgrade (OS, CD burning apps, DVD movie players, etc.) or a service bundle (switching ISPs, repair shop technician getting liberal, etc.). They'll put up with changes in interface and perceived backward incompatibility only because they think there's no choice.
There is no such thing as an upgrade that isn't both crippling and terrifying. That being the case, their suffering might as well result in a change for the better, not a change for the worse--or worse yet, no real change at all. The companies and people responsible for cramming upgrades down the throats of the ignorant masses don't appear to subscribe to this logic.