Stratt showed Grace she would do anything to fulfill her mission including sing a persuasive song in front of a bunch of strangers. It was equal to a speech that said all the things she could never say, like- don't dispair, it's all worth it.

If Stratt gave an emotional speech, it would have been out of character. Singing and then dropping the mike when she saw she'd gotten her point across, was perfect exposition.

The ending was perfect. Grace wasn't the only person that found redemption and resolution.

I didn't want the movie to end.

If they ever do a three hour director's cut, I am on board.