It is hard for Wenger to be sacked. He is making money for the club and for the investors with his cost saving ways. He does enough to get the Club into the lucrative Champions league every year but does not spend enough to buy the players who could help the club win something.
That is why players like Fabregas, Nasri and Persie realized that and left. Wenger is a company man.
Look at Moyes. He got sacked right after the Man U missed out the chance to play in the Champions League next year, when they lost to Everton recently. It is all about the money.