Grey's Anatomy shocking ending

As usual, Grey's Anatomy's last episode of the season has a macabre tone. Meredith, Derek, Yang, Mark and Arizona are all facing death.
What's more, Lexopedia seems already dead. Chance of survival? Pretty much none.

Why has Shonda not kept little Grey?

Drama for one, and apparently Chyler Leigh the actress playing Lexie, and Shonda have both agreed that her character has done pretty much all that's possible. The fact that they have secured the "big Grey", Ellen Pompeo for another 2 years could only help. The Grey legacy is secured. However Chyler will not leave completely the Shondaland. We will probably see her in Scandal, Shonda Rhymes new show, joining her Grey's Anatomy's father. 

The more endangered to succombe to their injuries are Arizona and Mark, maybe leaving Callie to raise the baby on her own.

April Kepner will be there, as actress Sarah Drew has already confirmed it. So has Kevin Mckidd playing Owen. The others will probably also stay, as they have extended their contracts.