[EVERYTHING] Quick Lord of Light Theory.

Jon Snow died, Beric Dondarrion died.

They are alive right now because the Lord of Light wills it.

They were brought back by the Lord of Light to help defeat the Night King because they both have a part to play.

Once (if) the Night King is defeated, they've served their purpose.

I think you get what I'm saying. I think they're living on borrowed time.