Are you suggesting Maguire and / or Van Dijk are better on the ball than John Stones? I think that's absolutely insane.
It's easy to play long diagonals out from the back. Anyone who's half decent with that much time on the ball can play a long diagonal.
And Maguire's runs out of defence, I heard those described by a United fan recently as "Like watching a Hollyoaks star get carried away at Socceraid".
The true test of quality on the ball from a centre back is how good they are at keeping the ball under pressure and how well they play forward through the lines in to midfield. Stones is the best centre half in the league at both of these things.