After seeing the first few minutes of the first episode i can’t help but wonder if that moment also was a factor for Aziraphale to act against the plan to end the world/universe.
He saw how Crowley more or less brought the universe into being (he even helped him - this world is basically their baby) and how excited he was and how sad when he heard that the plan is to destroy everything after only 6000 years.
This thing, earth and the universe are so important to Crowley from the very beginning and Aziraphale only starts to get it over time just like he gets to know Crowley over time. He grows fond of Crowley and the world simultaneously, those two relationships are therefore tightly linked to each other.
This means that earth and the universe are such a vital part of their relationship that it being destroyed, even if they both survive it, still means that a big chunk of their own selves get torn away. The sole existence of the universe and humans is basically the symbol of their connection. They‘re part of the reason why everything is and why everything will continue to be.
There is no them without the world and there is no world without them.