I didn't see heaven/angels so much as being infected by humans as Castiel realizing that there was no difference between some of his brethren (Uriel) and the humans - and then he got pulled in. The idea I came away with was: heavan, hell, earth - there is no difference (Infinite Diversity, Same Combination) as long as God is not present.

Or alternatively: Where the F is God?

I like your interpretation too. Gives me more thinky thoughts.