There was never any question of the characters remaining dead on SPN.. 😛 they will be resurrected one way or another.. it was only the actors' willingness to continue or quit the show.. plus what i as a viewer personally felt, i felt quite a few share that opinion. 😳
Cass's death was strange.. without any purpose whatsoever.. primarily done for shock value.. he may still be useful in the Nephilim track... and there was nothing from Misha to lose interest in the show.. maybe he was offered a hike in his paycheck.. or not.. that is another debate.. but the way, his death was written and enacted, left us desiring for more.. whether Alternate world Cass would be brought this side, or whether the Nephilim would resurrect Cass, is left for the makers.
Crowley, on the other hand... his death had purpose.. plus he promised the Winchesters something good- to help in closing the gates of Hell forever.. which would be a closure on the Demon story arc. whenever the Winchesters gain an ally, the ally's purpose in the story is over.. and they are quickly killed off. And the most important reason- Mark Sheppard called it quits. He was apparently not happy with something and hence disassociated himself from the show.. some articles are saying.. he had some grievances with the Creation conventions. So MS would have waited for the finale before disclosing that he would be leaving the show. 😳