Whether WN wants 76s (or any larger plane) is a rumor.
Why WN wants 76s (or any larger plane - if they actually do) is rumor and/or speculation
Why WN might want 76s in particular is also speculation. I like my idea, but I also appreciate rick's and Dominican's theories.
@Doors - could be. No, scratch that - you're right. MDW can't handle either a 739 or a 763, according to the published numbers.
I picked Midway because it happens to be their busiest hub - but the principle applies anyplace. More pax capacity without more gate capacity = bigger planes.