[bzzzt]

Why would they? Lots of people sell their old phone to pay for the latest model. Killing the resale value will decrease new sales.

[pasc1878]

Resale through Apple only -

Apple already will give you discounts if you upgrade some things.

So the resale value will continue albeit at a fixed price

[bzzzt]

They offer a pittance compared to the 'normal' second hand market. If people can't get enough for the phone the upgrade will not be bought.