I think your missing the point
Recently this year loads have been 80% or higher on lcy route.
Cityflyer are changing aircraft to increase service to Glasgow and Edinburgh that already have 9 and 10 flights a day return as it's more profitable.
Airport might not be able to put bums on seats but they can lower landing fees to match central belt airports and make routes more sustainable.
It's the same apd that needs to be paid by central belt pax only difference is the fees that the airport charges.
Glasgow at 176k pax on lcy route with over 3 times as many flights and given your calculations this would be a less than 50% load factor,
It's no surprise that since Edinburgh was taken over it has expanded way beyond what baa were capable to doing