Is there anywhere stating that Republic plans to put 138 seats on the CS300? I imagine if they're going to do stretch seating that we would expect more around 126-132 or so, putting it right between the A318 and A319. The Airbus fleet has a minimum of 900nm more range than the Bombardier and as much as 1,400nm more range, so I think it's fair to say the CS300 would be more of a large regional jet rather than a small mainline, it just doesn't have the range to match the Airbus A320 family.

Edit: Nevermind read the article again. Still seems to me that with the lower range you're just basically making it a short range mainline jet. Either way Republic needs to plan on keeping the A320 family around if it plans to operate some of Frontier's longer, heavier flights.