Erm. A little less of a stretch than the very sucessful MD82/83
The MD-90 was the ultimate overstretch of the DC-9 with lots of elements (starting with a 5-across fuselage) unchanged from 30 years beforehand, while others advanced.
The MD-90 was 4.5 feet longer than the MD-80 series, higher weights, more powerful engines. So it was the most stretched variant of the basic DC-9 design.
Not sure why 5 across seating counts as a disadvantage. Only one middle seat per row is really a plus for the pax. It would be a big improvement for the BAe 146.
