You are misunderstanding the situation.
Free Unlimited Supercharging on Model S vehicles will remain on all vehicles as long as they don't get traded in to Tesla. Once Tesla gets them back, they cancel that, which they are well within their rights to do, as the car is once again owned by them.
So, if you have a 2016 Model S with Free Unlimited Supercharging, that benefit will transfer to anyone you sell the vehicle to. The only time it will go away is if you or a future owner sells it (trades it in) to Tesla.