I am wondering if another point is around the number of seats on the different types.
The E145s and ATR 42s have around 50 seats whilst the Q400 has around 80.
I presume that in the same way as Flybe would struggle for example against EZY on BHX - EDI where the economics of an A319 wins over a Dash albeit with less rotations, the same applies between the Q400 and a 145 / ATR 42?
Obviously the ATR 72 has more seats so the maths would likely be closer on that equation.