Even if that were the case (big "if"), that can't go on forever. Even if it's literally
just a matter of who has enough data and training core-years, at some point a competitor, maybe with state-level support, will have "enough" data to come after you with their own self driving option. Perhaps now Tesla is at 90% (just say, it's just an example) and Mercedes (again say) is at 10%. One day Tesla may be 100% and Mercedes is at 30%, and the German government, for one, will be shoveling cash into the furnace the whole time if the German auto industry is in existential danger. The gap can only close.
Meanwhile once you reach full self driving, you're stuck at diminishing returns. Doubling your data won't double that: at best you could make a very marginal improvement to safety stats or range economy. Maybe with a lot of lobbying you could swing a slightly higher Tesla-only speed limit, but double self driving isn't a thing. Everything else in the car is commodity stuff like sound systems and seat heaters that anyone with a car factory and a team of designers and a copy of the Bosch Automotive Handbook can do if they want to. Probably you could get some network effects from the "Rent your Tesla out" app, but again, that's an option for anyone with a team of designers once the self-driving part is done and they'll be in the disruption phase and aggressively undercutting as best they can.
This also assumes that in 2029 you can't just use 80% (again... just a random date and a random number) synthetic training data and leapfrog nearly all of those hard-won physical miles. The costs of doing that will plummet too, so the training cost moat will only shrink as well.
Meanwhile Tesla will be ossifying as gigantic trillion dollar behemoths tend to do, accruing costs like pensions, management entrenchment will be in its third decade and so on.
On the other hand, if we're doing hypotheticals, if it did come to pass and Tesla is sitting pretty on a 13-figure valuation predicated on it being the only self driving car out there and a competitor pops up suddenly from stealth mode with a fully-formed alternative, much of the value could evaporate so fast it would be a structural hazard to the economy. That would be an interesting situation. And, remember, are that point it's all predicated on the software being the key, so Upstart Autonomy Inc. can license it to Mercedes and get it in cars almost immediately.
Is Tesla worth a lot? Undoubtedly. Is it that much? Questionable to me. But then the market disagrees, so what do I know (but also the market was off by 30% by it's own estimation so how much does it know?)