Two thoughts -

Boeing did pretty much keep the 767 line open without sales because they were planning on selling to the military. As it turned out many are picking up the 767 as a stop gap due to years of delays on the 787. But yes, you are right, the crystal ball wasn't there for the 757.

The 321 would drop into the ocean heading East to HI - At least it would not be able to have the reserves in needs that is for sure. The 321 is a pig in the climb because the wing loading is very high and the engines are marginal for the gross weight. I have never flown the 757, but I can tell you that the 321 is a big stretch for the 320 series wing and engines if one were to want transpacific or HI routes out of it. The 321 wing simply cannot carry anymore fuel because the weight will be too high for the available lift, even if the power were to become available.

Wonder what Boeing did with 757 tooling? It is very common with the 767 and also the top of the tube is 737, so it would be interesting to know if they could "resurrect it". I am sure they want to push the 900ER which has many of the same problems (if not more) of the 321.

787-3 back in the mix? Perhaps, but not until 2020 the way the 787 program is going......