I see some people questioning why Aziraphale and Crowley had to make their decision at the end when it seemed avoidable so here's my two cents on why I actually liked the finale.
Neil gaiman said season 3 was going to be what would've been book 2, and season 2 was the bridge to get us there.
They couldn't just do what Adam did in season 1 and rewrite reality, because season 2 was showing us that the whole problem was heaven and hell. The two sides making humans lives harder, while both having their own motives, the angels being hypocrites, etc., etc. All of this goes into Aziraphale and Crowley's decision at the end that humans need a world where they can be themselves without these external forces trying to sway them.
Aziraphale and Crowley knew this would mean they wouldn't exist, but their love of humanity was stronger than that. And further, their love for one another reflects their love of humanity. For 6000 years Aziraphale and Crowley have been on earth together, experiencing all the pleasures the world has to offer, including companionship. What I've always loved about Good Omens is that despite the main characters being two celestial beings, they are still so human, and a large part of that is their relationship with each other that makes them human. Whether it be platonic or romantic doesn't matter, whether they kiss or not doesn't matter. What does matter is that they loved each other so much and that made them human, and that love also made them love humanity.
Everything they've done has been for humanity, and while at first (book/season 1) it started as self preservation with selfish motives, it ended with complete self sacrifice to give humanity the chance they believe it deserves.
And! Those 6000 years aren't all for nothing! Those 6000 years is what gave them that mindset they had about heaven and hell and humanity. Living on earth, outside of their respective domains gave them a perspective that neither the angels or demons had - they knew what humanity needed. And that was for them to be out of the way.
And! Asa and Anthony meeting and living a happy life together is proof that if not for heaven and hell's interfering, Aziraphale and Crowley would have ended up together and gotten their happy ending. Asa and Anthony are proof that Aziraphale and Crowley's sacrifice gave humans the chance they deserve. People talk about how a lot of those 6000 years Aziraphale and Crowley had was suffering, but that is not the case for Asa and Anthony. Without heaven or hell, they can just be, like Crowley believed they could. Alpha Centauri was never a true option, heaven and hell still would've been an issue. But in this new universe, Asa and Anthony truly do get to be an "us".
And you know what? In their own universe, in their own time, with heaven and hell beating down on them - Aziraphale and Crowley still managed to find and love each other. And do you know what I call that? Ineffable.
