i believe the wings are design for a specific mission, ie what works for a 757 for flights more than 3 hours will not be as effecient as a wing designed specifically for shorter flights. Also the bigger engines on the 757 would cost more, the landing gears also as they are tall to enable the engine clearance.
Shortening aircraft has been done before.
The 747 was shortened which resulted in the 747-SP.
The A320 was shortened to produced the A319/318
The 707 was shortened and became the 720
Anilv