Ehhh, underwhelming indeed.
What I expected and would probably be a better ending:
Story started with foreshowing that "a saintess will appear" and she needs to "drive away the saintess".
Then we skip to the scene where she is called out for bullying the "saintess". Except she gets exiled for real. The devil laughs because he was expecting her to fail and now he can reap her soul.
However, turns out that she was real the saintess all along, and she deceived the devil and everyone else's, playing their misunderstanding to her benefit for the sole sake of her wish.
As a result, she gets out of a role that she has no interest in the first place, and gets her wish granted her wish.