This is correct, but only if you have the right drive-by-wire and sensor array tech. Essentially what you're saying is "Once you solve the hardware problem the rest is just software" which isn't particularly insightful.
What happens if Tesla can't solve the software problem because the sensors aren't providing the necessary data?
Which was my point. I'm intentionally making silly arguments that are valid from first principles, but clearly wrong in the real world. I'm sorry that wasn't obvious because I endeavored to make it so.
The point here is that Tesla have already sold the $10k FSD sensor pack to people and told them that one day in the future a software update will provide FSD, with no additional sensors necessary. Adding more sensors is not an option (well, maybe Tesla will do that for people who bought the upgrade, but it seems unlikely they'd do that for free.)