Crowley could not have used the Book of Life or his high level angelic/demonic powers to bring the world back because those things are part of the unjust system he is trying to end. By being a supernatural being, even as a demon, Crowley is in a relatively privileged position over humans. Dismantling that damaging, oppressive supernatural system means also giving up that privilege. As part of the system, literally created to be a tool for manipulating creation, he had to change along with it.
Using that power would be like making himself into a god, even temporarily. Holding onto even a small piece of it for his own benefit, or for Aziraphale's, would undermine the dismantling of it. So he chose to bring the world back through evolutionary, natural, and thus self-driven means rather than magic.
Yes, he deserved a better outcome, and so did everyone else, that's the point of what he did. That better outcome only can exist in a more equal world.
As for the human-created problems on Earth, he can't fix them as a superior being. He has no right to use his power to decide how to help, who deserves help, who he should listen to, what side he should take. He knows right and wrong, but it's not his place to take authority and impose it. Humans have to solve human problems. It's like the difference between a billionaire using their money to help people, which requires that they choose who gets help and what outcome they're aiming for, and a billionaire taking apart the political and economic system that allowed them to accumulate power-via-money in the first place.
I don't usually like stories about a hero making a supposedly selfless sacrifice because it often feels like an artificially forced choice. But I think part of what's so upsetting about this ending is that there are no real alternatives, and that has difficult and scary implications for the real world. Fixing systems of oppression means trusting that giving up privilege will still leave a place for you in the new world. But you might have to radically reinvent yourself to find it.
Crowley's one misunderstanding was that he and Aziraphale, as supernatural beings, were not already a part of the world and thus wouldn't have a place in it anymore. Or at least, he was willing to accept that possibility. But they did come back because no matter how much they may have felt like outsiders, their existence as people was just as much a natural part of the world as the humans they wanted to be like. In the end, dismantling unjust power and giving up their privilege also freed them to feel a real, equal connection to the world, just as they always wanted.
It's sad and upsetting and also beautiful and hopeful.











