That is an interesting problem for a corporation to have. Thier previous offerings are so strong that they actually compete with current ones. In our household it works in a kind of funny way. Wife really likes ithingies, with lesser enthusiasm by MIL and myself. So when WifeofZeugma is ready to upgrade, everyone else gets the hand-me-downs. Currently, she has a 5, MIL has a 4s and I have a 4.
I think it's amazing that the products are so strong that the previous products work just fine all the way down the line. When WoZ upgrades, I'll probably still be able to get a few bucks for the 4 I'm using now.
It’s actually a selling point for the new units that the old units retain value and functionality. That’s another hard-learned lesson that finally sank in with Jobs.