The thing is. The thing is, Aziraphale and Crowley absolutely were capable of sacrificing their existence for humanity. They show this quite explicitly near the end of the book:
"There are humans here," he said.
"Yes," said Crowley. "And me."
"I mean we shouldn't let this happen to them."
"Well, what-" Crowley began, and stopped.
"I mean, when you think about it, we've got them into enough trouble as it is. You and me. Over the years. What with one thing and another."
"We were only doing our jobs," muttered Crowley.
"Yes. So what? Lots of people in history have only done their jobs and look at the trouble they caused."
"You don't mean we should actually try to stop Him?"
"What have you got to lose?"
Crowley started to argue, and realized that he hadn't anything.
There was nothing he could lose that he hadn't lost already.
I felt a wee pang at the change in the show, when Aziraphale says “Come up with something!” instead of this moment of quiet bravery, because it’s my favorite moment in the book, and I love Aziraphale so much for it.
The thing is—they canonically (at least in book-verse) already offered to give their lives for humanity’s sake. With wings out and holding hands, might I add.
(While nothing could have made GO3 much better except an entire rewrite, couldn’t they at least have let them have their wings in the final moment, and snuff out holding hands, instead of a poorly-filmed and limply-executed finger kiss?)
Now I’m wondering why that scene was changed. Was it solely because whatsisname wanted to center Crowley and show that he had extra powers (hate that) and Crowley is David Tennant and power over time is a cute Timelord reference? That’s what I assumed at the time. Or was it because even then he was calculating the sacrifice of GO3 and didn’t want to repeat himself?
I can’t help feeling like that moment was taken away from Aziraphale, like more and more things are as the show progresses. I don’t feel that scene as written in the book detracts from Crowley, who gets the big predictive speculations shortly thereafter. It bothers me more now than at the time because now I know the scales will never be evened.
And I’m annoyed, because in my mind they already tried to make that ultimate sacrifice and it turned out they didn’t have to. A sacrifice that isn’t accepted actually counts. It was (to me) a lot impactful in the book.