He's a good backup and should be retained in that capacity. I think part of the offense's problems in the 2nd half is that he gets rattled under stress and his performance tails off. Unlike Luck who probably performs better as the game wears on as each snap grows in significance.
its something Brissett will have to learn before a team will provide a decent draft pick for him, so let him mature in a backup role. He's proven to be a solid backup, especially since he seems to play well early.
As an individual talent, Luck is in fact on a very short list. Ideally, when healthy Luck is probably the second most individually talented guy at the position, behind only Rodgers.
To the idea that he should be traded. The simple answer to that beyond it being logistically highly unlikely, is that it'd be a really, really dumb thing to do. I don't think Grigson would make that move.