What if the “license” to use FSDM belongs to the user and not the car? What if the original owner comes back and claims that he bought the license and he should be allowed to use it in his new car rather than go with the old car? Is that an unfair demand.

That is how it works for my Tesla. I bought one of the original 10000 or so Tesla and they came with unlimited supercharging. Tesla told me that I will have unlimited supercharging even if I get a new Tesla which I think is a better solution than it going away when I sell the old car.