Rewatching S10-12 and notice, it's funny to see that everyone just assumed if Dean is to die, the most desperate person who just wouldn't accept it and would save him at any cost and absolutely no bottom line is Sam; Sam would be more desperate than Cas and anyone else. Dean himself, Death, and Lucifer all think that.
That's why MoC!Dean asks Cas to kill him, because he thinks Sam would never agree to do it and would try to stop him, but he thinks Cas can. That's why when Death agrees to take Dean out of the universe, the condition is Sam has to die, otherwise Sam would try looking for him and saving him, but Death doesn't mention Cas. That's why Lucifer tries to force Sam to say "yes" when he tries to get out of the Cage by choking Dean, but he doesn't even really consider Cas when Cas is also there.
But all of them are wrong. It's Cas. Cas is more desperate, really willing to give absolutely everything to save Dean at all times, totally no bottom line for him, not for the world. Cas has made it very clear that he'd rather watch Dean kill the world than kill him (even though he promised Dean he would). Cas kills Billie to save the Winchester and is willing to risk the cosmic consequence. And Cas says "yes" to Lucifer so that Dean won't be killed, when Sam just refuses to say "yes", even if he knows Dean could die (if Cas didn't say yes and Rowena was a second too slow).
Back then, Zachariah commented that Sam and Dean are "psychotically, irrationally, errotically codependent on each other"; if he sees Cas now, he'd probably say really it's Cas and Dean being "psychotically, irrationally, errotically codependent".
