I strongly disagree with the Mourinho part, how could you even compare?
To answer the bolded question, it's because what you have to understand is that the board is an even bigger problem than Wenger is. In the first 5-6 years of the new stadium, they did not want to invest money in the squad, and wanted our finances to be directed mostly towards repayment of our debt. They actually gave Wenger a minimum goal of qualifying for the CL 3 times out of 5, in the first 5 seasons. So obviously the lack of ambition starts from the board, and since we are turning in lots of revenue, and Wenger is ok with not spending that much money, they're actually happy, because their goals are primarily financial not sporting. The worst thing to happen to our club is the board, they are the real problem here. That's why even if Wenger does leave, I doubt they'll actually invest in a top manager, that will demand funds to be able to compete.