People don't buy a BMW to hack it; you buy a car like that with the intention of having it serviced at the dealer only for a few years. You'd get caught the first time you brought the car in for service, and servicing it elsewhere lowers the car's value (as used buyers of these same cars expect the car to be dealer serviced.)
Maybe eventually on the used market it could get hacked, but that's some small percentage in the end... because a large amount of these cars are sold back to the dealer themselves, and then they go into "official refurbished used" programs.
It actually wouldn't shock me if you could end up paying less for that feature in the time you'd probably own the car than if you'd bought the feature. That's how they'll sell this idea.. "well if you own the car for 3 years which is your warranty time, you pay X, and the feature costs X + $500, you save!" They also might unlock it permanently once you reach the feature price... then they do the same thing to the next guy as the feature is tied to an account you have to cancel when you sell the car.