Apart from that I hear the 757-300 offers great CASM but is on the edge regarding stiffness, comfort and (de)boarding. 20 inches additional cross section could do the trick offering new cabin options and operational flexibility without introducing widebodies cost levels.
What do the 6 inches of extra width of A320 do regarding stiffness, comfort and boarding? The use of this extra width is at the option of airline - they can offer wider seats than on 737, or extra wide aisle. How does the performance of A321 compare with 737-900 and 757-200? (A321 has a small wing, so it cannot match the MTOW and range of 757, but it does match 737-900.)