I believe the major driver for the wider fuselage is so it can internally carry ordinance to provide for a stealthier aircraft.
It would have been better, however, if they had developed a family of aircraft with common avionics but optimized for the service and mission. So, the Marines get there lift fan, the Navy get folding wings and sturdier landing gear, and the USAF get a streamlined AC with better turning and speed. Again, they could have developed three largely different AC but utilizing common avionics and components.
If you ask me the thing that's going to make or break this is the missiles and the radar.
Ultimately, unmanned is the way to go and I suspect that we will never see the numbers they originally planned so the unit cost will go up.
Brian