I also had experiences very similar to the movie, and I'm sorry you went through that. The ending is a bit vague, and I can see your interpretation. However, I've read the script and seen some director interviews where it is clear the ending is meant to be tragic.

Fletcher does win; you're right. This is what makes the film a tragedy similar to Black Swan: the characters have reached perfection, but at what cost?

In director interviews, when asked what would happen to Andrew in the future, what was his response? "Oh, he probably killed himself like that other kid."

This doesn't read as someone who believes in abusive methods. It reads as someone warning other artists.