Look at the range the 70 seaters have now. What airplane has it replaced? UAL has put crj-700s on old 737 runs where mainline flew. So I do think there is a difference between 70 and 50 seats.
The 50's can barely make it above 30k and have all kinds of weight issues. They should be "feed" not destination flights: I.e. Sbn-Ord, pia-Ord not Sfo-Sat, or Lax-Dfw.