NW tends to be a bit id an "end used" of their aircraft. You really don't see any 7x7-x51s flying around after NW is finished with them.
It's actually not correct you never see former Northwest aircraft flying.
Their Boeing 707s and 720s
ALL continued for long years, like with Olympic Airways, Maersk, Cathay Pacific, LAN Chile, Kenya Airways, PIA, Alyemda and others. Actually a former Northwest 720 is still flying with Honeywell and six 707 with the Spanish Air Force, the Chilean Air Force, the Pakistan Air Force, BETA Cargo of Brazil, Omega Air and Allied Air in Africa.
Of the 727s, even about 10 -51s are still flying, with Kam Air in Afghanistan,
LA Suramericanas of Colombia, Clay Lacy USA, Marbiya Investments, Dallah Albaraka, Angola Air Charter, Heavylift Cargo, Flightstar, while many former 200s survive with Kitty Hawk, Mahfooz, Grecoair,
LAS, Sunworld, Custom, Champion Air, LAB, Capital Cargo
source:
http://www.airlinerlist.com