A great machine but less appealing to the accounting fraternity. For high density euro charters the A321 is much more cost effective , lighter structure from a smaller wing and later design techniques and a less complex smaller main gear.
In US several airlines have replaced their coast to coast &5s with new model 321s which can do the trip much less expensively now they can make it westbound reliably and the wider 321 cabin adds pax appeal. The 739 tho is a pretty horrible aircraft-a stretch too far from its excellent cousin the 738. It has the seats but as pointed out often cannot carry the bags of 200plus pax and it really is pushing the limits of the 73 design.
However the 757 really served markets that no longer exist as unique requirements, long haul out of LA Guardia, Hot and high from Denver , short difficult runways on Greek islands, busy US and Euro city pairs. When it came out the &% was perfect foe all these missions but like everything time has caught up and other aircraft -the 73 and A32 series can do 95% of what most 757 operators could do .
It is a shame as it is a great looking aircraft and a great sounding one if a pax hearing those cropped fan RB211 growl as they spooled up for take off and rocketed the thing into the sky like no other airliner before or since