While this is good for Chicago, these flights were possible due to a severe reduction in flights for St. Louis! American reduced 46 flights from STL, leaving it with only about 20 flights to 9 destinations. This is a huge setback for STL, as now they basically have no carrier hubbed there! (if you want to call what AA had a "hub")
Anyways, it should be interesting to see how UA reacts to this--considering they have gotten rid of most of their narrowbody fleet, and most of their hubs are being attacked by other carriers. But UA is currently serving many of the destinations AA has added, so this could play out well, and we may see some routes return to mainline service to compete with AA.