This will never happen.
The issue with this is that there are 20 year olds with Teslas and FSD. Tesla would owe this person FSD for life- for the next 80 years, on any Tesla they ever owned. They could purchase a used Tesla every 6 months, and Tesla would be required to do anything required to those cars to make FSD work.
This also assumes real FSD is ever going to be a thing. It very well may not, especially with anything that looks like current hardware. Right now there are couple hundred thousand people Tesla owes FSD to, but only on their current car. Maybe that will come to a head someday and Tesla will owe those people their $2K-$10K back. Maybe a billion dollars. That's at least a capped and understood value. But what if FSD requires $50K of hardware in the future, and they gave those 200K people "lifetime" FSD, and those people buy a used Tesla without the hardware...???
But the most interesting thing is that you are so sure that Tesla will get to "FSD" that you want to be more locked in to Tesla's autonomy, not less. History has not taught you that there is a very good chance Tesla will never get to L4 with anything that looks like a current Tesla? Tesla's path needs to be fewer promises about FSD, not "lifetime."
They need to just offer refunds and move on. Those that want them can take them, those that don't can wait it out on their current car.