Delta has a contract with SKYW for every aircraft in DAL colors. each tranche of flying (usually around 10-20 hulls) has a revenue guarantee for a period of 5-15 years. As long as SKYW meets contract mins in performance etc DAL has to pay them for the totality of the contract. All Capacity Purchase Agreements are like this. Now if SkyWest launches their own airline out of ATL in breach of some portion of their contract then there may be a legal basis for some form of compensation or modification to the agreement.

Anyway, bankruptcy, failure to met the performance metrics in the CPA or breach would be the only way out. If DAL decided to go all mainline tomorrow they would have to pay their various RJ partners handsomely to get out of the deals...