It wasn’t too terribly long ago that New York’s Mostly Mozart Festival was considered a relic, a musty and moribund example of what happens when classical music takes its audience for granted. But today, as the festival cruises through its 40th season, audiences are swelling, critics are cheering, and the whole enterprise feels like a winner again. So what changed? Over the last few years, “Mostly Mozart has become a haven for diverse, conflicting, even fringe-y viewpoints on Mozart and his near-contemporaries and descendants.”