Ok well let me enlighten you then.
The Q400 is actually cheaper to operate than the Q300 is - if you don't believe me, ask Bombardier! That is of course based on their lease rates and a comparable load factor. It is also quieter, much faster, and more fuel-efficient.
Why would you want to operate a 40 seater, when you can operate a 70 seater for similar dollars? How about freight, or the ability to stimulate growth on the route by using those extra seats for low-fare tickets? In other words, how about being a bit creative.
I was working for an airline with the Q400 in the UK, and after the initial problems SAS had (all long since sorted out), they were great. When I left, despatch reliability was 99.8%, and had been for some months. The Dash -200 and -300 fleet was being returned to Bombardier as rapidly as possible, because we found the Q400 was a far more economical proposition.
They regularly operate, full, out of London City (Docklands), and that is 1200 m long with a 5.5 degree glideslope, and a very testing departure profile.
The Do328 is at least as efficient as any of the aircraft being discussed as replacements, and is a hell of a lot faster, too. That is why it is so popular in Europe. It has, as the Q400 does, an active noise cancellation system in the cabin.
Both those aircraft will give you near-jet perfromance at a very low cost. The Q400, if I recall correctly, cruises at about M0.70. A 737 in economy cruise is doing about M0.72 (as is a 146 going flat out).
I'd rather fly any of the aircraft I mentioned, than doggy, noisy old 340s or ATR's.