Alternatively, they can sell the devices as today and provide a method to install any software the user wishes and even to remove iOS from the device. I guess they will be required to produce a basic documentation on all this so others can create the alternative services but wouldn't be required to modify their own software and services.
Anyway, maybe some depreciation calculations might be used to account for wear&tear but the core idea is that if you are selling something that works only on your other services by actively preventing competition you shouldn't be charging for the hardware that delivers that and the money you take upfront is just a fully refundable deposit to cover stuff like the consumer breaking the device or taking it but not using the services.
Early iPhones were sold only on special "iPhone plans", not because "special unlimited internet" (at least once iPhone 3G landed for sale outside USA and its shitty telecom market), but because there was a percentage paid from the plan to Apple.
If you control the software of the hardware you sell, you must refund with the inflation adjusted sums the users at any time they return their hardware.
This way, companies can choose if they are renting platforms they deliver their services like the TV boxes some cable providers will rent, or of they are selling products and the users can choose to do whatever they like with it.
The makers can still provide and and sell services on devices they built but they will have to be good at it.