I don’t think you realise that GitHub Copilot is powered by the GPT-3 Codex model so ChatGPT isn’t really competing with it. They are essentially the same thing.

That said I think GitHub Copilot and StackOverflow are complimentary tools. They aren’t really competing with each other. StackOverflow tends to be the resource that has the answer to some obscure and poorly documented framework feature. Copilot is more like autocomplete on steroids. They serve different functions.