Delta's recent decision to acquire 10 Boeing 737-700s configured with 124 seats each confirms my theory in the value of a 110-130 seat aircraft for the major airlines.
Previous Delta aircraft in this category were the Boeing 737-300 and 737-200, both were gained through the acquisition of Western Airlines. Delta later acquired more 737-300s from other carriers. Prior to the 737-200/300s, Delta flew over a hundred DC-9-30s.
While Delta may never fly hundreds of 110-130 seat airliners again, Delta's recent decision shows the gap between the 70-seat regional jets and 150 seat 737-800 is too significant to ignore.
I expect if Delta successfully exits bankruptcy and avoids US Air's hostile takeover attempt, more 737-700s will be ordered in the future.