Here's the primary reasons why the 727-200 was retired from service:
1. The life cycle of the airframe on many 722's are reaching way up there in terms of number of takeoffs and landings.
2. The 722 is far noiser and uses more fuel per seat-mile than the A320 and 737-800.
3. The 722 requires a third crew member, something that the A320 and 738 doesn't need.
The reason why Northwest (NW) managed to keep their DC-9 fleet in service was:
1. The fuel burn on a seat-mile basis was lower than the 727-200.
2. The planes are completely paid off, so no lease payments to contend with!
3. The planes underwent a major rebuild with 717-200-style new interiors and cockpit upgrades.
In short, NW
will keep their DC-9 fleet going until the airframe life and/or the new ICAO Stage IV
noise emission rules finally forces a phaseout.