The trend for Turboprops, RJ's, twin aisles, VLA's has been they get bigger, offering more capacity and lower seat mile costs. And the narrowbodies, they are just fine, nothing to see here, move on. ?!
IMO the writing is on the wall, unless you look the other way. Well, its still on the wall but ..
Boeing to consider a proper 757 replacement - Ghetto IFE
Boeing, Airbus Can't Replace the 757 - TheStreet
Icelandair’s model has withstood the test of time but 757 replacement dilemma poses a challenge | CAPA - Centre for Aviation
Airbus Studies 236-Seat A321
Sofar I found the posts arguing there will be no aircraft needed between 220 and 300 seat short medium haul unconvincing. The 757 production stopped 10 yrs ago and .. well that's basicly it.
Apparently Boeing isn't so sure and Airbus is increasing A321 capacity. Why?