The London Symphony is flying at a time when other orchestras are struggling to stay alive. Why? “You never stay successful by becoming institutional. Success is the result of keeping on the move, demanding more of yourself, challenging the very rules you live by. But at the same time, you have to be clear about what you exist for and where you’re going. If there’s one reason the L.S.O. has been successful, it’s because everything we do is for the music and without the compromise of `Oh, can we afford it?’ “