City should be a business airport with a limited size aircraft etc. etc.
So, only a certain number of business folk should be able to fly from an airport because... socialist central planning? If that's your philosophy on transport then just renationalise the airports and stop pretending to promote a capitalist market.
London City has a practical limit on aircraft size because of its geographically-restricted dimensions and steep approach, though an A319 has demonstrated non-revenue operations there. Belfast City can handle a ( lightly-fuelled ) 757 at a pinch.