Well in that end, the 747-300 had a place.
I remember those other versions of the 757. I think they should have made that platform a P-3 Orion replacement offering. It would have a lot more potential than the 737 based ideas.
I don't follow it all as much as I used to, and that is interesting about the 767-400. IMO the E-10 will most likely incorporate more than the glass cockpit. I like your idea on the GENx engine derivatives for these aircraft, but most likely that engine would be downsized a degree.
P3 Orion was built for low level sub hunting over the ocean.
The 757 could never do that.
Maybe the KC-767 would have a derated version, but the 767-400 is actually under powered. It could actually benefit from a more powerful engine. Ironically, the EU deliberately prevented the development of ETOPS certified versions of the GE/PW Alliance GP 7000 developed for the A380 for use in twin engined aircraft. Boeing wanted to develop a longer ranged version of the 767-400 using that engine. The prohibition prompted Boeing develop the 787 and to get GE and RR to develop the even more advanced engines that will be used in the 787 and the bleeded GENx derivative for the 747-8.