Just because HW4 is unlikely to be retrofit, doesn’t mean HW4.1, or HW5 or HW9.3 couldn’t be retrofit.
The reason HW4 is a different form factor is likely that HW4 is a form factor on which future HWx could be built: integrated, larger and easier to cool.
Chips get more powerful per watt every cycle of die shrink and gate size reduction. If Tesla wanted to, they could build a more powerful HW3.1 that would fit in legacy cars (or build two since S/X and 3/Y differ).
Why will they not do this? Because their first goal will be to make FSD work, and Tesla throwing better and more cameras and more processing power makes it more likely to work.
Now Tesla May decide it is better to refund, allow transfer or fight early FSD buyers, than to make their cars work. But maybe it could make the current cameras work with future HWx if it wanted. Or more likely retrofit the current cameras to work with future HWx on our old cars. Heck, there is even a very slim chance that the current hardware will be enough, once several years of new hardware has been thrown at the problem amd the software optimized.
What will happen? I dont think that Musk is going to stop over promising anytime soon and I don’t think people’s impatient nature is going to change. Lawyers aren’t getting less litigious, and the total value of unfulfilled FSD continues to grow. Should be interesting.