I dont know about that scenario.
Its putting the onus on the customer to cough up more cash, which is wide open to abuse by manufacturers.
Better the onus is on the manufacturers to make the cars affordable and competitive. If they dont deliver the customer goes elsewhere.
Its like the Ioniq 5 upgrade circus recently, some had enough of the shenanigans and went elsewhere
If Motability wants to innovate and offer its customers a wider choice, they could open the scheme up to 2nd hand motors, max 2 years old, something like that. You would certainly have Tesla’s, range rovers on the scheme and probably much cheaper.