As far back as I can recall(early 70's flying BUF-ORD on AA-707"Astroliners"),
there were few alternatives to UA/AA for westbound service. NW did not serve BUF and Eastern and (then) Allegheny had no direct service. You can
imagine the price-gouge; even for those now long-off days(for some reason, ~$175.00 one way seems to come to mind, but I may be wrong. Hey! 30 years ago is a long time).
Anyways, the recent AA pullback seems to be the ideal time for Southwest to cement its' position as the dominant carrier here. If they do initiate BUF-MDW with any sort of frequency, UA/AA can forget it. Why pay more $$$ to fly a CRJ/ERJ to ORD?