Isn't that the same thing as "not making mistakes"? If you can reduce the blast radius and didn't, wasn't that a major mistake in the first place?

A lot of code is certainly not written careful enough. Plenty examples of plaintext passwords storage in the wild.

No, it's recognizing mistakes will be made and no one is perfect. You will also make mistakes regarding blast radius - sometimes the blast doors don't close or function like you thought they would during an actual explosion.

Defense in depth is the idea. Swiss cheese model of failure.

Yeah, but this is bottlenecked by both your ability to observe and understand the system, which is precisely the thing that's being reduced by careless use of AI tools.