So you pay them higher to play better? You don't know how the motivation system works.
If you double their wages you get what? Nothing. He feels nothing at the moment that he needs to prove, now that he has this contract. You give them a pay raise once they deserve, and Marchisio was/is far from being in that category.
Your logic makes sense if we're talking about key players that are being wanted by Real Madrid or some other club. Well, you know what, Marchisio isn't that. He was just one of the players who underperformed.