One of the main reasons they went for the 717s as apposed to the A318 is with regards to their seating style.
2-2 A plane like the A318, and in your case, the 757, are meant for 3-3. Taking out two middle seats is not as cost effective as taking out just one.
Obviously with the DC-9/Md-80/717 2-3 seating, taking out just one middle seat is easy, then you get your 2-2 seating that they like so much.
If they weren't affraid of the 757/A320 width, they would have orded the A318. Thusly, I can't see them bothering with a 757 size plane for many years, if ever.
They're not handing trophies out today