It seems (longer) single aisle aircraft are substantially lighter and cheaper to operate than same seat capacity twin aisles. A 757-300 is lighter and cheaper than a same generation 767-200.
I think comfort has more to do with a soft seat with decent knee room, entertainment and recline.
If Boeing specifies a single aisle 10-15 wider than a A321, wide seats, aisles and bigger luggage bins I'm ok.
A321 Seats are already wider than 777/787 seats, just more width per passenger over the cross section.
A big single aisle by Boeing would open the door develop a real nimble, lean, optimized efficient, possibly hybrid 737 replacement in the 2030s.