having recently looked at the code for FTs in order to backout what i should rate nba players such that they shoot the correct percentage from the FT line, i can assure you that nobody shoots 0% from the line with an infinite number of attempts.
that being said, yes there is definitely a steeper part of the learning curve to FTs and it asymptotes... and this was by design otherwise why have FT shooting at all as a skill if there is a minimal amount of effect when you train for it or not. So essentially, yes, we have people be very bad a FTs when they have very bad ratings, but it is relatively easy to get them to be average FT shooters... and then harder to become exceptional ones.