AA has stored some 752s, including aircraft that are only 20 years old. The 738 is cheaper to operate. Unless the pax demand on a route is consistently more than a 738 can handle or the flight is longer than a 738 can handle (which implies that the fuel burn for a long, thin flight is justified financially), the 738 is a better choice. Blocking seats in a 752 is just a short-term workaround.