A little bit of thinking on FSD value proposition...
First, it's worth considering that for a lot of people, it comes with a HW3 upgrade (and a chance of future upgrades, if they're deemed necessary for FSD). And second, Tesla has started valuing it at 50% for trade-ins. So for a lot of people, it only needs to demonstrate (X * 0.5) -1000 in additional value. Or $5,000 for the current $12,000 price, and $6,500 for the $15,000 price.
And even if you ignore both of the above, consider that almost every other automaker has an autonomy program (e.g. Ford has Argo, GM has Cruise, etc), but none of them are even entertaining the possibility of letting customers own autonomous vehicles. The value of owning their own fleets of autonomous vehicles is way too great for them to even give a small piece of that away. So even if the probability of Tesla achieving L4 or greater is small, the future value is definitely large enough to make the current expected value greater than the opportunity cost of purchasing it.