I think ten years of mainline/express transactions have proven that carriers reject the OP's suggestion.
smb12 mentions CO selling off ExpressJet.
Delta sold Mesaba and Compass, and wound down Comair.
Alaska had Horizon as its sole express carrier and added SkyWest.
Carriers control the customer experience through brand standards, contract negotiation, and contract enforcement. Is the OP familiar with how few Marriott-family and Hilton-family hotels are actually owned by Marriott or Hilton?
It seems that AA/DL/UA like having multiple carriers to bid for flying. OP should check a good dictionary - I recommend Oxford American - for the definitions of whipsaw.