Why would an airline want to replace an aircraft that perfectly does its duties when the market prices are low, that is constructed to give 25 years + of sterling service and possible airlines are rare that would take them secondhand?
modifications make it apparent that the airline wants to keep them for some time still. QFs last 742s were retired at an age of 24. Assuming QF
wants to operate the 743s also until they reach final retirement time you´ll see them in the air until around 2010.
Egyptair is said to replace the two 743s they have quite soon. Recently they announced plans to dump them together with the pretty young 777s! These two Combis are quite young (15 years old) and will certainly find a buyer.
Corsair will start replacing their 20 year old 743s with secondhand 744s from next year on. They are high hour aircraft so probably bound for the boneyards.
Saudi Arabians 43s again will still see many years of service. Many of them fly on the "guest" workers shuttle in the Orient like the older 741s do that are also still serving after around 23 years flying for Saudia.
Air India two Combis have a high chance to fly many years on still.