It would be up to them to choose their models. If they want to keep the closed model where you can't use the device you purchase in absence of Apple, they will have to charge accordingly for the device and services and do the refunds if the user no longer want the device. I guess people would like yearly updates, so Apple will have to operate drastically differently to keep the closed platform model.

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.

So, the device is sold at a loss at 100 Euro but it doesn't work without mandatory services that cost 500 Euro per year?

Maybe, I don't know. Its up to the companies to do the math, who would like to pursue this model. If it's not feasible, the shouldn't be doing it though. AFAIK BoM is usually no that high, so it can be viable.

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.