Midwest Express has said in interviews that a plane with 3+3 seating reduced to 3+2 is not optimal. That leaves the 717 as the only plane still made that fits. They did say that they would not rule out the 3+3 planes.

The majors haven't even followed American's increased seat pitch so they won't take out more seats like Midwest Express. In theory, it's possible to charge a little more, cut costs a little bit, but it won't happen.