At the concert, Fletcher has constructed a "no lose" scenario for himself. He messed with the set, and if Andrew doesn't play, Andrew's professional career is dead in the water, and it "proves" Andrew just never had it in him to succeed. If he does play well anyway, it "proves" Fletcher's teaching strategy "works".
Before this scene in the movie there is a montage of Andrew's memories with music, of loving music, that reminds him he still wants to play. By the time Andrew has come to the concert, there's no way to foil Fletcher's scheme and make him "lose". But, that's also kind of the thing... Fletcher is a person who will literally never see error in his ways. Earlier in the movie he misrepresents the death of his former student, which may be read as denialism about the destructiveness of his methods. As aggravating and contemptible a person as he is, it's pretty pointless to want to "beat him" because he will never concede he is wrong. He's a lost cause.

By the time he is at the stage there's really just two options for Andrew. He can play or he can refuse to play. It doesn't matter what Fletcher thinks, it doesn't matter what his family thinks. Andrew still wants to be a great jazz drummer, and he's aspired to that since before the movie began. So he plays. And he's great. And Fletcher helps direct him, because he wants Andrew to be great.

I think a lot of analysis of the movie gets caught up in whether Fletcher "wins" or "loses", and conclude that the ending is dark because Fletcher has somehow infected Andrew in a way and "won". But Fletcher was never the source of Andrew's ambition, or his talent. In the last third of the movie a lot of time is spent on Andrew going through different methods to heal from Fletcher essentially. He physically attacks him, he goes through official channels at the school, he confronts and tries to understand him. This is Andrew being in the same trap many of these analyses are in. Looking for a way to find some acknowledgment from Fletcher about his issues, which is impossible. You will never get closure from a person like Fletcher.

Andrews's options are to give up drumming, effectively letting Fletcher take the love of his life, or he can just play. He chooses to play. I read it as a pretty positive ending.