This is actually a pretty important thing to understand. It can be better to rewrite than fix an existing mess. It’s similar to construction work in that sense: if you rebuild, you know what’s inside the walls.
That's why this copilot won't fly. The junior programmer will not be able to spot subtle errors but will kind-of feel "productive" by some random pastes from a giant brain, which cannot be interrogated.
If anything, I see copilot generating more work for existing, senior programmers - so there you have it.
Is that your main argument against Stack Overflow? Because for some percentage of people the use cases will be similar (that is, learning some quick ways to do something that they can then explore and learn about).
Sometimes resources like these are used as references and the solution is used as is. Sometimes they are used as a survey, and it's more like asking a librarian "I want to learn more about how to X" and having them give a short exposition and point out some sources to study. It's important not to let you view of it's usefulness as one type of resource bleed into your view of how useful it might be for the other.
When one gets snippets from Stack Overflow, he/she doesn't assume it is tailored specifically for the task, so extra check, modification are applied. With copilot people (eventually, especially beginners probably) will assume that the code produced by "magic" AI does exactly what they asked for.
Funnily this problem seems trivial when you have good test coverage for your code. Especially trivial compared to the insane amount of help you could get in writing boilerplate and simple logic from a magical tool like this, if it delivers on its promises.
Plus if you use this for anything non-trivial, it's functionally similar to using a third-party dependency, except the licensing terms are vague and there's no one to ask for help.
I could see it being useful maybe for testing. Even if it straight up copies code from the Linux kernel, you don't usually ship tests to customers so the GPL (probably) isn't a problem.
But the problem with testing is that you need to be 100% sure about how it behaves, and an AI generated test might not be reliable enough to be useful.
i'd argue the danger is even greater with tests bc the entire point of tests is to provide certainty. wrong tests are a false sense of safety that's unquantifiable without understanding every single line.