If an AI takes a copyright work and makes its own version-- say combining two novels by popular authors in a way that is unique but keeps large parts of the text intact, can I sell that? I think if I were the authors I would be unhappy.

Also, how hard would it be for copilot to include a comment saying "// I got this line from x repo" when you are copying from a new repo? I am guessing not hard at all. Then at least the user would be aware of where their code was coming from and could be expected to make a judgement. If the line is "let a = b" then probably no worries. But if it is hundreds of lines of a simulation, all from the same repo with no changes, then I think some attribution is good for both parties.