It's not just the seat width - 'narrow body' aircraft simply feel small and claustrophobic - especially that long tube when they get long enough for over 200 passengers. Plus, loading/unloading over 200 people with a single aisle takes a long time which hurts turn times. I personally hate flying coach in a 757 (and I worked on the thing) - if I can't end up in first class I avoid flying 757s (same thing with 737/A320 when the flight is over a few hours). A cabin wide enough for twin aisle hurts drag and weight, but it opens up the cabin and makes for a generally more pleasant passenger experience (even with the same seat width/pitch). It also allows far more cargo room and even makes many maintenance tasks easier (I've spent time crawling around in the 757 avionics bay - and I mean literally crawling - by comparison I can stand upright in a 767 avionics bay).
That's why Boeing is probably going to go with a twin aisle one the MMA.