With the uncertainty surrounding Luck, he needs to be kept at least for 2018. By then if Luck has shown to fully come back with the same arm strength and ability he had before his injury, then you can see what the market would look like for Brissett in the 2019 offseason. If a team is willing to give up at least a mid to high 1st round pick, you contemplate the offer.
It's all about judging how you think Brissett will progress. If you think he has maxed out as a player, you trade him while his value is as high as it will be. If you think he will continue to progress, you keep him since his value will only go up.
But right now he is needed on this roster until Luck comes back fully healthy and Indy has a quality OL to keep him that way.