Yes, UA could do that with IAH, but the fact is, UA combined with CO to get IAH and EWR. Hubs require a substantial amount of O&D traffic to work and they work better when they're located in a favorable spot. The Metroplex (DFW) and Houston (IAH) are huge metro areas, have substantial O&D and are growing much faster than the rustbelt. They both have geography on their side, as they're not too far from the middle of the country, so they don't require much out-of-the-way flying for efficient connections.

Chicago's importance to UA and AA may decline over the long-term, but the 10 million people in northern Illinois, northern Indiana and southern Wisconsin will still be there and will provide substantial O&D for decades to come. For 30 years, UA and AA have split the legacy customers with two good-sized hubs at ORD and over that time, they have watched WN move into MDW to capture a large number of CHI's domestic traffic. And of course, both UA and AA have become smaller at CHI. But I don't see either one abandoning CHI or their Texas mega-hubs.

Now that I write this, AA will probably announce a complete de-hubbing of ORD over the next few months.