You can't be a hub with only O/D traffic. It goes directly against the hub and spokes model. The whole idea for a hub is to have as large a network of connections as possible, once you reduce an airport to 'profitable' flights to significant markets you no longer view it as important to the logistics and efficiency of your airline and thus no longer a hub.
I think that point has passed some time ago with UA and LAX. I consider LAX a hub in name only. Given that LAX is a highly competitive market just for its O&D traffic, it probably doesn't make a lot of sense for any of the majors to make it a true hub. Makes more sense to zone in on their respective fortress hubs where margins can be higher.