Yeah, the more I think about it, the clearer it is that this--the part where the earth isn't 6,000 years old (and a Libra), where the dinosaurs aren't God's little joke that humans haven't figured out yet, where there was never a Plan, and never a Fall, just humans being humans in a universe that exists under rules we can learn to understand--is always how it had to end.
If you want to imagine Aziraphale and Crowley still out there somewhere as an angel and a demon--God only said She'd create a universe the way they asked for; She never said it was the only one She'd create. Maybe they're also still in the bookshop, writing their universe back into existence. Maybe She also put everything back how it was, like she first offered, and they're an angel and a demon doing shenanigans on an Earth that's connected to Heaven and Hell by a giant elevator.
If you want that for them, you can make that happen, in your fic or art or your own private imagination. The Good Omens universe doesn't have a God anymore, and your free will is as good as anybody else's.
And in the universe of my free will? It would be an awfully contrived coincidence if, after 18 billion years, Aziraphale and Crowley just happened to reoccur and reunite in our time, and I don't believ ein cheesy coincidences. So it happened a bunch of times.
The first time a proton and a neutron came together to make an atomic nucleus? Them.
When two atoms bumped into each other and stuck to make the first molecule, also them.
First time some hydrogen atoms and an oxygen atom found each other and made water? You know that was them.
When bits of space dust started being pulled together by their own gravity, into an accretion disk that eventually compacted into a sphere and became a planet called Earth? They were in there somewhere.
When amino acids did a little dance and somehow ended up making a new thing called life, they were there.
They were probably a few molecules of algae a whole shitload of times, because Earth's algae era was pretty long, but eventually they got to be part of the same colonial organism, and then they got to be plants, and fish, and dinosaurs, and some of those enormous dragonflies they had back in the dinosaur era, and then eventually mammals, and apes, and eventually humans. And they kept finding each other. Not because God planned it that way--although she probably did shove them in there, on the principle of "waste not, want not"--but because the mechanics of the universe allowed it to happen, and so it did.