Curate, connect, and discover
Thinking about Dean's motif of corruption. "The very touch of you corrupts". Heaven, and Dean himself, see this corruption as rot. Hellfire. Moral decay, and, inevitably, destruction.
However, "corruption" can have other meanings, especially when defined by spn's (or most modern versions of) Heaven. "The very touch of you corrupts" when Cas first touched Dean, something took root. Dean's love, his protectiveness, the rugged life he was more or less forced into, his stubborn insistence, against all odds and in all circumstances, to prioritize family, food, hearth, a safe place to sleep and a planet that won't fall apart on him. Those roots spread- but not in any evil sense.
Tree roots.
Strong and unrelenting and vibrantly green, fed on tears, soil, blood, ash and salt, and on red meat and love. Wriggling through the cracks in Cas' celestial being, curling around the divine light and leaving rich earth behind it. Is it corruption? Is it decay? Yes. But nothing, angel or demon or antichrist or anything else, that Dean touches can help but become a creature of Earth. Most humans don't notice, humans are already Earthly beings, and most monsters are as well. Angels though?
Heaven and Hell are two sides of the same coin. The opposite of both is Earth- Earth with no care given to an afterlife, kindness and love existing for its own sake, no hypotheticals. Dirt, rot, messiness, food, hunger, tiredness, laughter, love.
and that scares the SHIT out of Heaven
they've seen angels fall, either living as humans or truly Falling, their grace charred and covered by hellfire
they've never seen one turn into a physical being before. even an angel reborn as a human is still an angel. Cas isn't becoming human. He's still an angel, but-
if the other angels can see the roots piercing through his true form, churning through grace like a sapling in new soil
what other word would they have but "corruption"?
(obligatory SAM IS NOT EXCEMPT FROM THIS. HE'S ALSO WEIRD THIS WAY follow-up bc yall will just hate Sam for no reason istg)