You tag Dak this year and it costs you like 27. If you could get Brady done for, say 20, that's like 7 mil over what all of the Pay Dak fan base is willing to pay, either way. I mean, your gonna have to pay a backup QB either way right? If you keep Cooper Rush, you are gonna have to pay him 7 to 8 hundred Thousand at a minimum because he's a 4th year player and probably more because he plays QB.
If you can do that, then bring them both in and let them compete for the job. Let the best guy start and the other can back up. Draft a QB to develop and lets do this thing. If Brady is truly the better QB, start him and win games. Let Dak back up and learn. If Dak wins out, then Brady will want out anyway and he will work with the Cowboys to get to a team where he can start. This way, you give the team the best possible chance to compete for a championship IMO.