Agreed. The -321 might hold the people but cannot carry the cargo or fly from hot/high altitude airports like a 757. Even on takeoff the plane is miserly. It has so much performance that the engines normally are between 80-86% N1 even on heavy flights. They will stay there until level off that's somewhere between FL370-400. That translates into longer engine life and reduced mx costs. Boeing got it right in spades on the plane.