Believe me SFly, by the time Boeing admits that some of their product line is past its sell by date it is waaaayyyy beyond its sell by date.
Its a bit like an Old VW Beetle, you can stick in a new engine and a new instrument panel it is still an old Beetle
Before either the 757-300 or 767-400 were launched most analysts within the airlines did not believe in them because of the residual value problem and performance wise. Boeing had to continue with the -400 because of the threat of the A330. If you look at the performance of the the 330 (particulary the -200 (the -300 is a dog) ) you will find it the ideal DC10 / MD11 replacement or for routes with slightly higher loads than the good old 767-300ER. The only takers for the -400 have been operators who had Boeing in their fleets. Even KLM a good boeing customer went for the 330 next to the 777.
Believe me the next to go is the NG. Eventhough sales are good for the moment Boeing realises that they need to start quitely working on a replacement. They will not tell the market as with car sales as soon as you find out that there is a new model coming residuals and orders for the current model will fall.:o