I agree it's not simple but the catchment area of an airport most definitely isn't defined by the surface access of an airport, it is defined more by choice and availability. Take for example flights to somewhere like Tenerife. Flights to Tenerife are available from airports throughout the North of England often with choice of more than one airline therefore the imaginary catchment circle ring will be significantly smaller from Manchester airport than if you compare Tenerife with somewhere like Dubai where somebody in say Leeds will have to choose between Manchester or Newcastle for their departure airport.
Just as a look at the catchment area I've picked random weekend to fly to Tenerife in November.
Liverpool - (3 flights)
Friday & Saturday - easyJet
Sunday - Ryanair
Manchester - (22 Flights)
Friday - Ryanair (2), Jet2 (3), easyJet (2), Tui (1)
Saturday - Ryanair (2), Jet2 (3), easyJet (2), Tui (1)
Sunday - Ryanair (2), Jet2 (2), easyJet (1), Tui (1)
Leeds - (5 Flights)
Friday - 2x Jet2,
Saturday - 1xJet2, 1x Ryanair
Sunday - Jet2 x1
Newcastle (7 Flights)
Friday - 2x Jet2 1x Ryanair,
Saturday - 2x Jet2
Sunday - 1x Jet2, 1x Tui
I suspect there will be more flights for next November with Jet2 providing 3 flights over the weekend from Liverpool alone. In total 37 flights from the North so the catchment area has plenty of availability, flexibility and choice. Catchment area clearly can sustain this.