It's actually the eleventh busiest airport in the USA (by number of passengers) in 2011, behind ATL, ORD, LAX, DFW, DEN, JFK, LAS, SFO, PHX and IAH:
http://www.airports.org/cda/aci_common/display/main/aci_content07_c.jsp?zn=aci&cp=1-5-212-218-222_666_2__
Yes, it is the sixth busiest by number of aircraft movements, but that's irrelevant to the economic viability of a hub at CLT:
http://www.airports.org/cda/aci_common/display/main/aci_content07_c.jsp?zn=aci&cp=1-5-212-231_666_2__
CLT's deficiency is not the number of passengers - it's that too high a proportion of them are connecting passengers and not CLT O&D passengers.
As to the stat of airport movements, add together JFK-LGA-EWR, then you will come up with the busiest movement area in the World.
Just sayin'
Again(at the risk of repeating myself),........ "IF" one was to be able to cherry-pick the best of AA and the best of LCC, The result would be a hypothetical airline just as strong as DL or UA !
It IS what it IS !