Do you suggest that hardware manufacturers would make more money from the small cut they get from a customer buying 5-6 new applications for 4-5 dollars each than they would make if they force you into buying a new $500 handset 6 months earlier than you normally would? I don't believe this makes sense.
Nokia and Apple sell hardware; the apps, stores, open source, is just the bait to make their hardware more attractive. I am sure that when Apple come up with how to change the iPhone (the upgrades we've witnessed are insubstantial compared to what NITs have gone through, for example), they will abandon backwards compatibility.
OTOH, software companies like MS maintain compatibility to an unmatched extent.