Mesut Ozil first signed for Arsenal because he wanted to keep a promise. He had promised Arsene Wenger that if he was ever going to be in the need to look for a club outside of Real Madrid C.F. he would contact Wenger. He kept his promise.
That day another promise was made. This promise was made by the Arsenal FC board and Arsene Wenger to Mesut Ozil that they will be competing for the Premiere League along with the Champions League. One can argue that they kept their promise but in all honesty Arsenal qualified for the Champions League every year but has failed to win either trophy despite finishing in the top 4 of the Premiere League for the past 20 years.
Mesut Ozil took a chance on Arsenal during a time no other top player was willing to come to Arsenal FC. His signing filled the fans with belief and was a statement of intent to other teams as well. It showed Arsenal FC is willing to splash the cash and that there are many more players to come. It is safe to assume that Alexis Sanchez signed from FC Barcelona, who has also stalled during the contract renewal negotiations would not have come to Arsenal if a World Cup Winner like Mesut Ozil was not already present.
If Ozil is sold then it shows Arsenals inability to keep top players. He needs to be rewarded for his loyalty and decision to take a chance on Arsenal. I feel that Arsenals loss of Ozil would be much greater than Alexis Sanchez leaving as it can open the door for more questions on the boards mentality and desire to win by the competition and players.