AA's contract does not allow anyone other than eagle to operate any jet with more than 50 seats. CHQ was paying AA a fine of several hundred thousand dollars per month when the Yonited 170's first arrived, hence the abrupt purchase of S5 to get around that contract violation.
And that is why contracts are not FOREVER! Contracts end, and re-negotiated all the time. AA contract may not allow this now, but you can bet your a$$ that little clause can disappear in a blink of an eye.