If US airlines buy new aircraft again they will do it the same way as they have done it so far. AA, CO, DL will go on to order Boeing only, UA and NW will order both and US and HP will probably order only Airbus. There is no reason to change, as most airlines have long before decided which way to go, which types to order.
UA won't replace all A32S with B737NG, and NW, US and HP won't do that toor.
Me & You & a Plane Named Blue...