Ellen's been dragging the show's quality and reputation down with her greed. She has more than enough money to support her children and family. If she hasn't been challenged creatively all these years then why continue to do it? GA could've went out with a satisfying ending for the majority of beloved characters and couples - including MerDer, but she chose to feed her ego instead.
Despite being the show's namesake, Meredith Grey was never as much of a lead as say Olivia Pope and Annalise Keating. The show has always been an ensemble. Meredith was only as good as the people around her, which is why she used to be a much better character than she is now. And Ellen clearly wasn't a great supposed-leader all those years if she let things get so toxic. She is part of the problem.