>But the new Codex checks and corrects itself?

Only syntactic errors, not functional.

>just enough to replace the majority of entry level positions.

Codex is very far from that. I think, you have wrong idea what juniors do. Even the juniors have understanding of the parts of the system and they cannot be instructed by few sentences. Most of their work, is actually trying to understand, what is wanted from them - which computers are really bad at still.
There are layers of understanding, it will take leaps in AI, before it can easily reason about high level abstractions and implement it in code.
Also, the "programming" is not perfect information game. You have to ask LOT OF questions and debates, before you and the client converge on what they need.
I am sure, it will come, but right now, nobody is using it for real work.