Boeing and the "Middle of Market Airliner"
Since Boeing stopped building 757s and 767s (at least passenger 767s) they have been studying some form of replacement. It has been reported that United can't wait for a new Boeing and, presumably, they've looked at the A321LR and the A330neo and decided they don't meet their needs so they are thinking about new 767s.
So here's my question. Airbus has updated and re-engined A32x, A33x and has made noises about A38x - the "neos". Boeing has constantly re-engined, stretched, added wingtip fences to 737s. Why not do a 757 max or 767 max? What is it about those two airframes that seem to make that not possible?