You say that, but then what exactly is Willians role? If it is to defend, then i'd far rather Mikel. If it is to attack, then his stats don't stack up against Schurrle's. If it is to act as a box to box midfielder, then his stats don't stack up against Oscar or Ramires... In short what's he good for?
Willian versus Mikel (Mikel not as quick and offers nothing with the bal at his feet going forward, all this despite his potential as a youngster)
Willian versus Oscar (Oscar is not a player that gets up and down the pitch easily like Willian does although Willian is not as effective as Oscar attackig wise)
Willain versus Schurrle (Schurrle can't defend, would we really have conceded the late equaliser to City if Willian was still on and picking up the run of Milner)
Willian versus Ramires (This is the closest comparison but Ramires has no technique at all, Willian has a least some and does know how to beat a man one on one with ease, although his end product needs improving)
Also Willian is the quickest over ground in my opinion in our squad barring Hazard so he has that and if Willian's role was to be decided, it is like the jack of all trades but master of none, he can do what Mikel, Oscar, Schurrle, Ramires etc can do, but not as great as each one does their best.
Frankly Ramires, Mikel and Schurrle have had plenty of opportunities, especially Schurrle to oust Willian but has failed every time.