The problem with the 757 is that it's too costly to produce. They were never really designed for efficient mass production. Designing a "revised" 757 to compete with the 321-neoLR is either going to have to be clean sheet (expensive, risky) or re-use the old blueprints and inefficient processes, also expensive, hard to sell. If Airbus can undercut you by 50% for the same market, Boeing won't bite. IMHO it is more likely that Boeing will introduce a "mini" 787 to compete instead of reintroducing the 757. Take out some fuel tanks, shorten the fuselage, voila, a 757 replacement.
RE-ENGINING THE BOEING 757 AND ADVANCED TECHNOLOGY -AirInsight