It is a little more strategic than that. Look at DTW. No city is declining faster or further than Detroit, yet DL is maintaining its presence there, killing CVG instead. While Detroit is shrinking fast, it is still fairly large and it is well positioned geographically for some routes, particularly since DL is mostly shut out of Chicago.

Even Chicago is depopulating (although the suburbs are still growing) while UA, AA and WN duke it out. Las Vegas is falling on its face but it is still one of WN's largest stations.

AA was not especially interested in STL to begin with since it was inherited from TW. It is far more important for AA to keep its position at DFW and ORD, which serve similar hub functions. If AA did not have ORD, than STL might still be a hub.