These two adorable beings:
didn't choose who they are
don't like their jobs but have no choice but do them
fell in love with humanity
have gone through A LOT of pain
have relied on one another for millennia
admire qualities of the other
understand they live in a world that does not understand them
and wants them harm if they step out of line
so they keep each other safe
and their best is not exactly the same because they are NOT the same person, and shouldn't be
they are perfect for one another
because they complement each other so well
they fill each others gaps and cracks
and understand each other's pain
Aziraphale and Crowley are not opposites. They are assigned to different sides but their love has always tied them together into their own side. They BOTH know this. The whole idea of GO is that an angel who is a little bit of a bastard and a demon who can't help but be good, are just two beings falling in love. Exceptional in how common what they have is (or should be).
However, from their point of view (yes, Crowley's too) they have no real way of escaping their world. They have tried, sure, temporarily; they kept their wonderful relationship going as far as they dared, but they both knew it could not last.
From our point of view, we know that events of S2 set them up for even bigger conflict than stopping Armageddon did.
And they will win again. And become free.
In a Universe that was set to be written down in a Plan, a Book, a prophecy... none of this matters because THEIR LOVE changed and will change everything.
Aziraphale was not wrong for going to Heaven. He did what he had to. Crowley was not wrong for staying. He also did what he had to.
And from this point - they will reach their ultimate goal and that not despite the fact that they were forcibly separated, but because of it.
I have no idea what will happen in S3. But I know it won't be Aziraphale profusely apologising for being a moron and thinking things that were meant to be set in stone can change - the thing that Crowley showed him is possible.
F15 is not what either of them wanted. In any shape or form. There can't be a right choice to make because there was no right choice to make in F15.
But it is what they were forced into and what will become a catalyst for changing the whole structure, the whole basis of their reality.
And I don't think it will be just for them. It will be for everything and everyone.
