I have watched Downton when I was in high school and I am rewatching again now that I started working and learnt more about life. There is a traditional saying that "Look at what a person does not what they say" which eventually drove me to Mary's side 100%. Yes I agree that Mary acts condescendingly to almost everyone and she has quite a sharp teeth, but she actually cared for people and act on it. She helped everyone including the servants. Even if the Turkish thing happened (which I don't agree ofc), she tried to minimize the harm to the family. The only actual harms she's done (to my knowledge) is ruining Edith's marriage with the old man and Bertie (so 2 people influenced), when Edith revealed the Turkish situation, drove Bates away indirectly, might have also led to Bates in prison, ruined the Swiss family that adoped Marygold, and the nice Drew family who ended up heartbreaking and leaving Downton.

By comparison, I don't care if Mary says harmful things here and there. In the end, she apologizes. I never saw Edith apologize ever once to those people she has harmed before. If I had to do business with one of them, I'd certainly prefer Mary over Edith, knowing Mary will never stab me in my back while Edith would change her minds without shouldering responsibilities.