The show was originally "EP's" show. EP was hired first, and for Shonda, the show was clearly about Meredith. But then Merder and McDreamy became the engine behind the show's breakout. (Not taking away from the others actors, the music, the clever writing or anything, but I don't really think you can dispute that. Merder were the "headline" in most of the mentions of the show.) And suddenly to the media (and the majority of fans), EP and PD were "co-leads." We've heard that that caused resentment with IW. But what about EP?
I think she was happy enough to go along with PD being perceived as "the male lead" for awhile. She was getting paid good money, the show was thriving, she had a life outside it, and she didn't have to sit through a million inane interviews, etc. And although that was not the way Shonda wanted it, she clearly exploited Merder to build her empire.
But with Scandal and HTGAWM, you saw what I think Shonda wanted for EP, and what EP maybe has wanted for herself. KW is clearly "the lead" in Scandal; it's THE OLIVIA SHOW: no one mentions it without talking about KW and she's credited with the lion's share of it's success (what's left after Shonda gets credited, anyway.) It will propel her career beyond Scandal. Same with VD on HTGAWM.
EP says she doesn't want to act after GA? Well, maybe. But I do think she'd like a little of the glory that KW and VD are getting for carrying their shows. And she was never going to get that while PD was around, no matter how lamely Derek was written. (I'd argue she was never going to get that while SO was around, either.) But Shonda finally has the power to make it happen, beyond the objections of ABC and PD. She's taken the show back, for Meredith and EP.
I still think EP leaves after S12 (or maybe S13), after a season where she gets a chance to taste the same kind of glory that KW and VD have.
But how do you generate the same buzz for a show that's so old? Hmmmm... Maybe through a splashy and controversial exit for PD?
Just a thought.