The economies of the 757 started to look unacceptable compared to the 739 and
A321.
No, the 757 ceased to be produced because the commercial/marketing departments of major airlines, SIA for one, back in about 1985, didn't like single aisle and the baggage loading system of the B757. Twin aisle considered essential for a good cabin service on short sectors with a full load, containerised baggage required for quick transit pax transfer, with bags and also for a quick turn round.
Regarding the range of the B767-300ER, have frequently flown Manila-Bahrain with a full load, into a constant headwind, plenty of fuel to spare.