A lot of the busier airports these days prefer to have larger aircraft and will charge accordingly. You only have to look close to home in the UK to see this.
London City - Airlines operating larger aircraft
Gatwick - Airlines using small aircraft are being charged more to entice them to use larger aircraft (Hence Flybe withdrawal)
Heathrow - Capacity restraints resulting in only real increase in airline capacity can come from aircraft upgrades.
Looking at the suggested routes below
Brussels - Struggles to work and has failed by Air Wales and Eastern Airways
Paris CDG - Will work at the right times with the right aircraft type
Jersey - Currently still covered 5 x weekly (Summer by Flybe)
Cork - Will work at 1 x daily
Guernsey - Last attempt operated by Blue Islands 1 x weekly and couldn't even fill a J31.