If there was ever any doubt that a conductor can revitalize a struggling ensemble, Louis Langrée is putting it to rest with his debut this summer at the helm of New York’s Mostly Mozart festival. In retrospect, says Anthony Tommassini, the fault for the festival’s notorious struggles in recent years must be laid at the feet of Langrée’s predecessor, Gerard Schwarz. “It’s sad to have to say this so definitively, but Mr. Schwarz, though a tenacious defender of the basic concept of the festival over his 17-year-tenure as director, was just wrong for the job. He lacked a compelling artistic vision and was too limited as a conductor.”