fundamental reason why sastiel would never work is because cas is a giant gaping yonic abyss of pure hunger and sam has like an hp lovecraft level fear of yonic abysses. dean on the other hand subconsciously wants to return to the womb because it was his last place of true safety so him and cas work very well together.
this shot in particular is gonna make me fucking cry from laughing so hard. i cant believe y'all were screaming dean winchester when he looks like THAT compared to bobby. mr ankles your lunch break is over go back to your big sky set.