The MD-90 was the ultimate overstretch of the DC-9
Erm. A little less of a stretch than the very sucessful MD82/83, a far nicer aeroplane than the 737 in most respects.
I'd suggest the demise is more to do with the similarity to the 737 figures. I don't think Boeing wanted it, but it was too far down the road to kill it outright.
Maui