Simple matter. A plastic 737 will be cheaper to operate and maintain and more comfortable to passengers. Combine it the new engines being developed and you will have 737's flying to Europe, further lessening dependency on the outdate spoke hub system. In addition, the 737 interior is very cramped. The A320 is wider and much more comfortable.
I'd say it's a safe bet that the 737 replacement (single aisle) will have a wider fuselage.