PD's exit seems like a lifetime ago, and really was in terms of planning and preparation, so I think that no longer has any effect on why the writer's room is so unfocused (funny that Derek, such a "minor character," still gets referenced).
Too bad we can't go back and peruse the old TWOP forums because we'd see the exact same comments and complaints. People wanting more Alex (year after year), not understanding why there's no focus on this character or that, people frustrated that the cast is bloated (Mercy Westers, anyone?), Shonda's focus is no longer on Grey's (Private Practice, anyone?), unplanned pregnancies (almost every pregnancy!), marriages crashing and burning, focus on characters that no one but Shonda really loves (Denny? Penny? They rhyme, that's funny), implausible stories (Meredith's never-ending supply of surprise sisters).
"Insanity: Doing the same thing over and over again and expecting different results."
It's not going to change. It's never going to change. The show will go on to season 20 and there will be more unplanned pregnancies, more couples getting together and then divorcing, more characters, more focus on Meredith the sun, more fan frustration because they tell, they don't show.
And yet...it'll be season 20 and viewers will be hanging on, hoping.