All things considered, Cas just looked plain dead, and frankly, given that he was stabbed via angel blade by LUCIFER, I’m inclined to think that it might be best for the story to keep him dead.
This time, however, there wasn’t a last minute save from God or even a disappearing body to leave the possibility open.
Admittedly, Cas has seemed pretty definitively dead before, only to pop back to life.
His dead body was very much lying on the grass, and Dean at least was clearly convinced that his best angel pal was finished for good when he stared down at the corpse.