The same United scouts have been been helping with the recruitment the last eleven years. They have given us some gems like AWB, Maguire, Antony etc. I won't go back to trusting them so swiftly just because someone has bought the rights to run the football department.
Again, who says that he is one of the most promising CB's in the world at 21? He is doing well in a team that plays park the bus hoofball football under a very negative manager. Maguire also looked good when he played his football at Leicester, who employed similar tactics. Passing numbers never tell the whole story unless verified with the eye test. For example, as you mentioned, Maguire's passing numbers are not bad but he crumbles whenever he is pressured on the ball. To become a team that is good at playing out from the back we need someone is good when pressured and can pass the ball well between the lines. Nothing suggests that Brainthwaite is that guy.
If we want to remain a team that plays back to the wall counter-attacking football then I can understand going for Brainthwaite. However, if our plan is to become a progressive team that is good at keeping and moving the ball then we need someone better.