Just finished Good Omens 3 a little bit ago, so heres my thoughts :)
First of all, I know people are upset/disappointed at this ending, I get it but frankly I feel like it sort of makes sense even if it’s not what we wanted. I don’t think Crowley and Aziraphale wouldve ever been able to live how they wanted, something would always drag them back into some big thing.
And I don’t believe they wouldnt have been able to leave Earth either like Beezelbub and Gabriel because they cared too much about humanity to fully leave it behind. So for what we got and for what they chose, I do believe that they made the right choice in the end even if they had left all that they fought for in the end.
There’s also something poetic about how a gay couple was able to choose how they wanted to go out and refusing how the world wanted them to be.
Besides, the ending just shows us that no matter the universe they will always find each other again and be able to live happily without the guilt or fear of heaven and hell. There will always be an Crowley and Aziraphale one way or another.