As has been posted before, US is not currently a full member of Star Alliance, as its partners have not invited US to participate in the immunized joint ventures with UA, LH and the other partners.

If US and AA combine, it's a near certainty that the combination would remain in the alliance where the much larger airline (AA) is already a full member, including the immunized joint venture, with its transatlantic and transpacific partners. That alliance is Oneworld.

Among other problems, one reason that US is in poor financial condition (despite all the bankruptcies and wage-slashing) is that it is the only domestic airline that must compete WITH its alliance partners across the Atlantic. It has no Pacific presence and, thus, no antitrust immunity with anyone across the Pacific. If the immunized joint ventures benefit the partners who have been granted ATI so they no longer must compete with each other, US is the beneficiary of none of those benefits.