“A door has not exactly been slammed shut. But the Barenboim years in the Windy City were not marked by a mutual affection that might have left both sides with a more enduring relationship. Barenboim’s departure, made public two years ago, came by mutual consent: he, frustrated at having to combine musical stewardship with the glad-handing of potential donors; they unsure of his commitment to audience-building at a time of falling subscriptions.”