I really doubt that ChatGPT will be able to give the kind of guidance that turns juniors into seniors.

Getting juniors un-stuck on "simple" problems, maybe. Stack Overflow already does this. Image doesn't build? Search for the error message online. Don't know how to build the old software that only works in Ubuntu 14.10? Sure, you'll find that.

Suggestions on how to refactor, proper design of interfaces, what skill to acquire next? Maybe, but that will be a bigger surprise.

I think it could go either way depending on the product. For example in app/game/web development where code quantity > quality, hire more juniors who can bust out code with ChatGPT all day. But if you're developing software for medical devices, vehicle control systems, HFT, etc. Then nobody's going to let some college grad using ChatGPT touch it. You'd hire senior engineers who can be responsible for the reliable operation of the software and they can use ChatGPT for code review, test suites, etc.