Have you considered that as Elsa is "letting it go", she is freezing her homeland and killing lots of innocent people?

I've never understood how that song became the "anthem" because it really isn't the moral of the movie. The moral of the movie is the Troll's "fixer upper" song--that no one is perfect, everyone has room for improvement, and love can help them change.

In this case, I hope Few doesn't "let it go" but instead recognizes the imperfection and tries to change. Hopefully he has loving people around him to help him do that. But "letting it go" is definitely not the answer for him.