Airplane design depends on engine availability, as Lockheed was reminded early in the L-1011 program. The second consideration is where to put the landing gear. All other compromises follow.
I was SLF for over 3 hours in a Horizon CRJ yesterday, and that size tube won't replace the 737/320. Horizon long term plans reportedly include dumping the CRJ in favor of more Dash 8-400, which burn a third less fuel.
GB