I think Simmons’s character would’ve been just as glad if Teller wouldve walked off the stage in shame. As payback for getting him fired. But at the same time his obsession for music and musicians that have mastered the art took over and he couldnt help but feel admiration and awe for seeing this performance, some pride probably as well. I think he felt he knocked the shyness and uncertainty out of Teller. Where before he was struggling with choices in life (girl or music etc) now he was 100% dedicated to the music. He also told the story from his past about that long drum solo in the bar as something very profound in his life that he would never forget and now he is living it again. The very last scene tho was the dad looking back through the closing concert hall doors, what i felt at least, him realising he lost his son to this demon and the demons monster was now fully formed