How bad could it be, famous last words. But in this case, the argument's solid. Dean's still alive in Jack's timeline, enough that he's considered family, a caretaker. Jack didn't mention any current evil they're fighting (there's always something), it's distant enough for them to be child-rearing. No apocalypse world, no Mark, no primeval darkness.

"You're right. An answer exists, and we've dealt with the creature, at home." Cas breathes, almost disbelieving. Shaking his head, he looks up and around at the others. "The Mark is a seal on a cage, and it exists both here and there. But only you were brought through the portal, Dean. If we remove it here and not yet there, what difference would it make to the darkness in another dimension, that still has an intact seal?"

Cas pushes off from the wall, pacing as he brainstorms. This impossible problem has a solution, definitively, and it lifts an ominous cloud from the entire affair, like a gradually tightening noose cut free. Now, they get to work. "The spell to bring you here duplicated the seal, but the Darkness isn't inside Dean. When I possessed him, I would've sensed an ancient evil, but there was only the corrupting influence from the Mark."

Maybe not the best time or company to announce he'd possessed Dean earlier? Cas doesn't leave time for any questions there, quickly moving on.

"What about Abraxan magic? Do we have any allies in Thorne who'd be willing to check their resources for anything we can use?" He turns to Sam, because there's no point addressing this to Dean. "There's a whole world of research out there we haven't touched."

A deep inhale puffs out Castiel's chest, and he almost feels excited.

"We can fix this, Dean, we just need you to hold out against the Mark long enough."