Tesla don't price in any value to FSD in their residual value estimates in their PCP calculator, and none of the 8+ lease companies I spoke to before deciding to buy outright did either. It was a straight addition to the total loan amount, with payments increasing over the term accordingly.
Let's be clear - robotaxis aren't going to happen in the Model 3s lifespan, regardless of anything Elon says. It has to be at least 5, more likely 10+ years away technically, and probably another 5 years on top of that in regulatory terms. There are a myriad of significant technical hurdles to overcome even if you assumed the Model 3 hardware (resolution, CPU speed, framerate, number of cameras) were capable of it, which I doubt, and without considering the various things that already might and do cause it problems (low sun, mud, etc).
So, yes, right now you are buying in to the fact that you'll be on the bleeding edge of whatever self-driving tech Tesla releases (UNECE stifling notwithstanding), but assuming it'll be anything more than that, even in 2-3 years time, is wishful thinking I think.