Many answers to be noted. Thanks again for your time.
Concerning the "spamlining" strategy, I won't use it. I like to play more "RP" than just to reach the top of the ranking. I want to make a profit to keep playing, not to break records. Even though I've accepted (for now) the scam model and the salary drop.
Otherwise... A lot of changes. I've understood the supply and demand thing a bit better.This means I'm making more profit, especially in places without competition. I have also started routes to the Middle East, Eastern America and North Africa. These are the most profitable at the moment.
I didn't go for the A320s but for the 737s. It was cheaper. I now have 7 of them. The advantage is that they are leased. So I could change them in 3 years by not renewing the contract?
Concerning the other aircraft. My A320 is on the end of its contract and will be replaced by a 737 to keep the same family. I have my ARJ for another three years. And my ATRs are purchases... Do I really have to sell them? Can't they be used for short-haul transport to the north of Ireland or Scotland?
Well, if you've got your money situation under control, keeping them around/getting rid of them won't be a critical issue anymore, but I'm not sure that it's really gonna make much money either. It's just that each aircraft family has a base maintenance cost to it, i.e. having 1 747-400 and having 100 of them will charge x dollars per aircraft in addition to the base maintenance cost of the 747 family itself. You can still keep them around if you want, I just never personally use anything smaller than a CRJ705 because... well, it's just a lot of hassle finding routes for them when an A319 or E190 even can make 5x the money with 1/2 the effort.
Regarding 737 vs A320, generally the 737 variants are cheaper, but they tend to have slightly more fuel burn, a bit less range, are smaller, and slightly more runway requirement (compare B738 to A320 or B739 to A321). The A320 aircraft are more expensive but I just enjoy using them since their variants come earlier (A319LR and A321 v.s. B737-700ER and B737-900ER). Towards the mid-game and end-game though I operate both families, mainly A319LR/A321-200 alongside B738. It's really more of a preference type of thing, you will likely use both families as your airline grows larger simply for the reason of "more planes = more routes = more profit" later on.
I usually lease aircraft for 10 years at a time and basically hold onto them... forever, lol.
Finding new routes is one of the biggest things to running a successful airline, that way if someone comes along and sets up base at one of your hubs you won't lose all of your routes.