Wizz / Blue Air / Czech are ...
at Liverpool because they have decided it is the best option for them to serve the North West market
Yes, as your premise is flawed. The airports you list and the process being followed in general by Ryanair in these locations is one of shifting from secondary to primary airports which serve the same urban centre.
Now, just re-read your two statements quoted above. When you refer to Blue Air, Wizz and Czech the catchment area is deemed to be "the North West Market". But if the name in the frame is Ryanair that same premise is flawed ... according to you.
From an airline perspective, the name on the airport door is irrelevant. Issues of civic pride are immaterial to them ... let local councillors worry about all that. The airlines just see markets and catchment areas. No two airports have identical catchment areas (indeed, catchment area varies by individual route), but some have very significant degrees of overlap. Whether you, I or anybody else dislikes the fact, LPL and MAN are two such airports whose catchments do overlap significantly. That is what the airlines see, and they couldn't care less if it hurts the feelings of individuals consumed by passionate civic pride.
Ryanair do recognise that LPL and MAN do not serve identical catchment areas - it is likely that they will maintain certain routes such as DUB and certain mainstream Mediterranean resorts from both - because each airport can sustain a standalone service. But there are also routes which Ryanair will perceive as serving a larger catchment area, and they will operate that route from the airport which offers them the highest potential for profit. We can all see the way the wind has blown in that respect.
Now, you claim that moving flights from Girona to Barcelona, Bergamo to Milan or Modlin to Warsaw is "quite different" from moving flights from LPL to MAN because "they are airports which serve the same urban centre". Well, the City of Girona is actually the capital of its own province (also called Girona). Its airport primarily serves that city, but its catchment overlaps that of Barcelona and Ryanair has used it on that basis. The city of Bergamo has a metropolitan area population of almost 500K and its own top-flight Serie A football club, Atalanta. Its catchment area significantly overlaps Milan, but it is not Milan - it is Bergamo. Modlin Airport is located alongside the county town of Nowy Dwor Mazowiecki; there are many miles of green fields between this town and Warsaw, but the catchment area overlaps significantly and Ryanair uses it accordingly.
Meanwhile, the distance between the airports of Liverpool and Manchester is 30.7 miles by road. Completely consistent with the examples listed above. Distinct cities, yes, but with airports serving a catchment with significant overlap whether we like the idea or not. Just like BGY/MXP; GRO/BCN; WMI/WAW and others. Ryanair treat these two airports accordingly in their planning, just as they do with the pairs outlined above. So my premise is not flawed. You just don't like it! If in doubt, read your own quote about how Blue Air, Wizz and Czech see things.
I believe it is reasonable to infer that if airline charges aren't going to rise to pay for the investment then the increased revenue needed will come from the other main cash cow, the passenger.
But if an airport business succeeds in in growing its customer base it will have the ability to spread its costs amongst more passengers meaning that the need to increase charges can be avoided.
Finally I would ask you to bear in mind that a number of the contributors on this thread rely on the airport for their livelihoods, so telling them 'not to take things personally' could also be perceived as patronising.
The concept of relying on an airport for one's livelihood is not alien to me! But most folks who work at airports are well aware that they are working for a business which competes with neighbouring businesses for some of the same customers. I don't see why staff should need to be sheltered from that reality. Airports compete with each other for business all the time. That is not about to change regardless of whether it upsets people or not.