Wenger had declined from his heady days of the 2000's, but even his worst was far cry from Mourinho's or plenty others in big clubs. He had to operate under shoe string budgets, year in and year out, compared to Mou or Pep or Klopp or most others in big clubs across Europe. Also, massive money and commercial pressure have complicated the player-manager relationship, which he had to tackle very late in his tenure.
Earlier, Sir Alex used to wield the cane like a school master, but now its all player power.
Prima Donna's like Pogba, Ozil, Sanchez and others take in fat contracts and then play hooky, bringing the organization down.
In these circumstances, it seems Wenger did ok, working within limitations.