Elon sees FSD happening between 2020 and 12020. There is no know reason that it can't happen. Well let's assume if we work real hard we can do it in 5 years instead of 5000. If it takes 15 or 50 or 100 years, no matter. That's basically the same, but the earlier we start the sooner we have it.
Of course, there are those who think there is some spiritual/religious component to true AI that we'll never be able to simulate. But Elon either disagrees or things that level of AI isn't required for FSD.