I think the problem nowadays is that the car buying public (both new and pre-owned) expect vehicles, even low in the model/trim range to have certain bells/whistles as standard.
Things like sat-nav, cruise control, air-con, Bluetooth etc are seen as ‘must haves’ by most. No doubt over time Apple/Android Car Play will be considered an absolute necessity, particularly by the younger generation (who seem permanently welded to their iPhones etc).
The other difficulty is that for the scheme, a totally basic car would not command the second-hand value of a car with bells & whistles. So, Motability would get less to re-invest into the scheme (in theory anyway) when they sell on/auction the vehicle.