Frankly, I don't know what brilliance led Boeing to shut down production in the first place. The 757 may well be one of the most versatile airplanes ever built.
Unfortunately, sales of 757s to airlines did not reflect the pax preference. I much prefer the 757 over 737s even though the fuselage cross-section is the same (cramped) Boeing tube first offered in the 1950s.
But the 757 has a mid-cabin lav (or 1+2, depending on config), I feel like the beefier long-haul wing produces a better ride, and fairly frequently they use door 2 for loading & unloading, which probably speeds things up a bit ... or at least I don't see the slightly happier pax in F as I board.
The fact that the 757 could do longer sectors tends to mean its fuel burn per pax is higher, especially on, say under 2,000 mile sectors. Add in that the 757 has older engine tech, heavier landing gear, etc, and unfortunately for all us passengers, the -800 and -900 will be the 757 replacement for a chunk of flying.
I look forward to the A321s. At least they're coming with the 18 inch seat. On 6 across, it will help, IMO.
Edit: The big drawback of the A321 is range, so it won't replace the 757 for TATL point-to-point flying.
Edit 2: Airbus reportedly is considering an ER version of the 321neo, aimed at TATL. We'll see if anyone bites.