Very true, at the extreme cases of range and capacity, the 757 can outperform the 737. But experience shows that airlines don't always operate their aircraft that way. I lost track of the times that I flew on a 757 from London to Brussels, Edinburgh and Amsterdam, for example. On the European routes, that was a two class cabin as well, so no where near maximum capacity, and I would bet a 737 could have done the job just as well.
Boeing 757 can fly missions that are simply out of range for 737.
Indeed. But how often do they?