The loss in revenues, especially for high-cost short-haul tickets, would have more of an impact on the balance sheet than mileage liabilities. After all, US's "liabilities" under DM can be discharged entirely at any time, for any reason. They could just announce DM is discontinued and all miles are worthless -- and it would only cause a bit less angst than this truly, truly boneheaded decision.
DM was what set US apart in a good way -- but now it's turned into a joke.