A Purely Selfish Action
I've had the show running in the background while I've been working and this line tripped across my ear and made me realise this is how/why Aziraphale is going to save the world.
He knows the Second Coming is on its way and this time he's front and centre for the ride. The last time it was Armageddon (Antichrist Edition), Crowley appealed to Aziraphale's love of human things to get him on side: food, music, shows, nice wine.
Yes, Aziraphale has other reasons for standing his ground as well, but when it comes down to it, he is selfish. He wants to keep his things and his people safe and comfortable. He likes to be around his people and his things and being promoted to manage the Second Coming has cut him off from all of it. From his bookshop. From his books. From his little part of Soho. From his coffee shops and restaurants and the Ritz. And, most importantly, from Crowley.
There's a lot of mess surrounding their final interaction, both of them speaking at cross-purposes and actively not understanding why the other is doing what they are doing, but there can be no mistake that there is the love there. And I have absolutely zero doubts that the Metatron used Crowley as leverage against Aziraphale in ways we didn't see - there's a reason we were only shown parts of that conversation.
In Aziraphale's mind, if they're on the same side, then Crowley is safe. He's thought that since S1. "Heaven will win, of course". "You were an angel once". When Crowley rejects that, it is precisely a rejection of that - Heaven and angeldom and stepping back into that rut. Crowley makes it crystal clear that he would be happy with just Aziraphale. The love is there, from both sides. Instead of one saying "come away with me" we got the double-whammy "come away with me".
I'm absolutely certain that being stuck back in Heaven is Aziraphale's worst nightmare, especially without his emotional support demon. And the bigger the threat of Armageddon 2.0 grows, the more he realises what he will lose.
I don't doubt that any threat to Crowley will activate every Guardian instinct in that fluffy angel's head. Aziraphale has spent many a happy time letting Crowley rescue him from dangerous situations, but if anyone - anyone - threatens harm to his demon, Aziraphale Will Not Care For It.
I am so here for the territorial fierce Guardian of the Eastern Gate who puts his foot down and says "No, actually, this is my world, that is my city, that is my demon, and you are not destroying them, thank you very much. If you want to get to them you have to go through me and thank you very much for giving me all the powers of the highest of Archangels :)"