First off, I will always side with the worker over the employer. If Maguire wants to stay, he is 100% entitled to the stupidly large contract United foolishly gave him. No arguments about that. What he isn't entitled to is a pay off to move elsewhere. He can stay and have his current contract, or leave and get whatever contract his next club is willing to give to him, and that's it. United owe him nothing in that case.
Also, I believe that if he stays, he is either purely motivated by money, or delusional. There is no universe in which he wins back his spot. He will never be a first choice (or even second choice) defender for Manchester United again, much less the captain. He is completely unsuited for the style of football we want to play, and he's not going to suddenly become faster, more agile etc at the age of 30. He's only going to get worse. I realize that in order to make it in sports you have to have incredibly high self-confidence, but this is ludicrous.