I don't think it's one or the other. You want them both to be able to without the worry of leaving the space. You want more options. A quality CDM allows Maguire to step forward, it doesn't mean the CDM can't. One of Maguires strengths is coming forward with the ball.
Also I don't really recall Maguire ever really being exposed and left for dead. I feel that weakness is exaggerated and it hasn't really been a problem despite his lack of speed. The whole defensive unit has been awful which includes those infront of him. A pacey CDM and CB next to him opens up more options.
A fit Bailly, Tuanzabe or Koulibally type player is perfect next to Maguire. The energy of Somoure, Fred/McTominay and Fernandes with that defence would be unreal.
I'm an optimist when it comes to United.