If there are no bugs, you're right. However, every bug that does ship becomes harder to debug, and so does every bug you fight with during development. The impact of bugs becomes more severe and less predictable on average (dangling reference or buffer overflow can affect pretty much anything.)

The more accurate analogy is seatbelts' utility in a car fitted with various collision avoidance devices all the way up to fully autonomous driving. I say you'd still want the seatbelt for when collision does occur after all.