But then, because the universe is elastic in that way, you can be like, actually, it doesn't matter. Castiel can, for all intents and purposes, cease to exist for the span of an entire storyline, and that won't feel off the pace at all.
I know lots of people would cry plot hole, or inconsistency, or whatever. But I actually unironically really like the strange uncanny of that. XD I think it's cool. The universe doesn't have rules; only guidelines!