Airport Catchment Areas
Just a friendly reminder arising from the very interesting discussion developing above. We are again starting to fall into the trap of presuming an airport's catchment area to be some geographical district we can draw on a map with easily defined boundaries. This is very far removed from the reality.
For example, the catchment area for a MAN-DUB service is very different from that for a MAN-SIN service. A customer who would happily travel from the North East to board a MAN-SIN flight would rarely consider doing so for a MAN-DUB journey. I'm sure the reasoning for that is quite obvious to readers on here so I won't elaborate. But it is a truth that every route/service has a catchment area unique to itself due to all manner of influencing factors. There can be no one-size-fits-all definition of an airport catchment area.
Additionally, catchment area is not purely a geographic phenomenon drawing customers to their closest departure airport. There is also an 'economic catchment area'. Some customers will travel further than others to secure a lower-priced flight or one which offers them loyalty rewards. Two next-door neighbours can both make perfectly logical decisions to book flights to the same destination from different departure airports. One may prioritise price, the other elapsed journey time. Neither is mistaken in their choice.
Using my own situation as an illustration of this principle I will use an example quoted above. I would not contemplate booking LPL-Zadar as I consider the departure point too inconvenient for my own needs. However, now that I know MAN-Zadar is likely to be offered I'm actively checking my available summer travel dates. The fact that the two departure airports are 30 miles apart does not mean that their attraction is equal to any one customer with unique circumstances (and every customer is unique). I value time and travel convenience very highly; I recognise that others hold different priorities. That is what makes the catchment area unique to each individual route and service. No airline will know the actual demand until they try a route in practice. They can only make a forecast (guess!).
Another factor often misunderstood is that many customers use a thought process beyond the obvious. In my own case, my start point is not: "I wish to visit Zadar". It is actually: "I wish to visit an interesting short-break destination which is new to me and conveniently available from the departure airport down the road at an attractive price". MAN offers a selection of options which fit these criteria at any given time and I make my travel choice from that list. Now that Zadar is an option, I may choose it over Gdansk or Tallinn (for example).
Always factor in that airport catchment areas are a profoundly complex topic which can be analysed right down to the preferences of each individual customer. Airline professionals are well aware that switching route resources around comes with consequences attached in both markets affected. Something we often see with Ryanair is a tendency to 'freshen up' its destination choices from time to time. For example, I visited Tours with them (from MAN) last summer. Very nice too. But having visited that city I'm now looking for somewhere different. If Zadar has become available it is a strong candidate for my next booking. Meanwhile, someone elsewhere who has already visited Zadar may be delighted if the chance to easily visit Tours is put before them. It is very smart to keep updating the 'menu' of short-break destinations offered even if the based fleet and overall number of weekly departures remains unchanged. (Note: I'm not suggesting that is the case with Ryanair @ MAN this summer. I await their final fleet deployment plans with interest).
Catchment areas. Darned complex subject! Don't be tempted to over-simplify!