This is my reaction too, but five matched aircraft might be less bad if the share pilot qualification requirements with the A319/A320 and also use many of the same parts as well as procedures and qualifications for mechanics.

At the time the CAB was disbanded and airlines in the USA were deregulated, it was claimed that all of the economies of scale could be realized with only six matching airplanes. However, we don't seem to see many new entrant airlines with only six aircraft, although perhaps that can be partially explained by economies of scale in reservation systems and the need for market power in negotiating contracts with suppliers.