Hubs Closed - approximately last 20 years

AA - BNA, RDU, SJC, STL
DL - DFW - (CVG and MEM have been scaled back although they are still considered by DL hubs)
UA - None
CO - DEN
US - PIT, DAY, LAS

In terms of Chicago, keep in mind that WN has a very significant presence at MDW which keeps significant pricing pressure on UA/AA at ORD for the markets where they compete. In other words, right now there are three large airlines vying for a major chunk of the Chicago market. With another round of consolidation likely I think its hard to envision Chicago will always have three big airlines in a big way.