America is a tough place to be a music director, and even some of the world’s greatest conductors eventually decide that the constant strain is just more work than it’s worth. Case in point: Mariss Jansons, who this weekend conducted his final performances at the helm of the Pittsburgh Symphony Orchestra. “Many things didn’t work out the way he would have liked or else Jansons wouldn’t be leaving so soon — after only seven years. But he is a class act, a committed servant of the music and above all a good person.” Jansons leaves the PSO an ensemble transformed, but with an uncertain artistic future.