I get really annoyed when a lot of Grey's fans complain about Shonda ruining the show by getting rid of characters, but the truth is that most of the characters that died or left did so because the actors playing them wanted to (April and Arizona aside, that was a recent development and I've been seeing this argument without their characters for years). E.g Lexie, Mark, Callie, Stephanie and Cristina left because they wanted to. Burke and Izzie left because of issues with behaviour and they're conflicts. Derek left cause of both. Shonda didn't wake up one day and decide to kill off peoples faves. As for relationships, Calzona, Slexie, MerDer were all supposed to be end game but if one half of the couple leaves, what else is she supposed to do?
If actors want to leave to do work on other projects, Shonda has no choice but to work about it. So stop acting like it's not a rarity for her to fire characters. The only examples that come to mind are George (who was also unhappy but is still on good terms with Shonda), Teddy (who left for financial reasons but has still been brought back) and April and Arizona (who like I said, are recent actions and haven't left yet so have nothing to do with prior complaints.
Ben and Addison left for spinoffs so that doesn't count. And apart from that, the other missing characters were minor.
TL;DR: Don't blame Shonda when actors want to leave to dp other things. They aren't her prisoners and she can't make them stay against their will.