By the time the organizers of the U.S. Open told the world that it too would be expanding to a 15-day event, the only appropriate reaction seemed to be: of course it is. There’s a Grand Slam arms race under way, and keeping up — whatever that means — comes at a price. Costs rise every year. Tennis players expect more prize money. So if a major like the U.S. Open sees a pot with tens of millions of dollars right there for the taking, given all the additional tickets, media rights increases and ma