I'll address only last point because everything else stems from that.

> Tooling that prevents the bugs in the first place is effectively free once it has already been made.

Surely you understand that such tooling is basically impossible to create and even if it was impossible it would be so horrendously expensive and hard to implement that AI would actually turn out to be cheaper. Under the false assumption that such tools could be produced you are right, but that assumption is absurd.