The ending was appropriate. Everything in his life was a complete mess. His daughter hated him, he had no other family, poor health, lived in a trailer, and he was eventually stripped of the one and only thing he had left, wrestling. He literally had nothing and was at the end of his rope. He tried to work a regular job but could not cope. He made the decision to continue wrestling (before the girl came to him) even though it would kill him because it's was the only thing he had left and the only thing that brought any happiness into his life. If you look at his face in the final scene you can see how broken down he was; to me it looked like he was crying. His intent was to die wrestling - this is evident as his opponent told him to take the win a few times but he refused knowing he was on the verge of a heart attack.
It all made perfect sense to me. Great movie.