PAXboy,
I used to commute significantly in/out of BHX.
With LH it might have been a CRJ to DUS, a BAe146 to MUC and an Airbus or Boeing to FRA.
With LX it might have been a BAE146 to ZRH, a BAe146 ZRH/MXP yet an A320/321 ZRH/FCO.
With KL, something like 5 services a day, it could have been a F70, a F100 or one of two varieties of B737.
These are just examples of different sizes for different niche routes, try and put an Airbus, Boeing or larger Embraer etc. on everything then some routes simply wouldn't work.