I understand some people don't want to change certain routes if they can't find a plane that fits the bill for a replacement but why am I seeing people flying DC-6's into 2000 whilst also having a large 737-800 subfleet, by that point with the amount of competition the aircraft shouldn't even be turning in a profit. Are people that lazy for changing fleets..
I have a airline that's decent sized for example, I started out with 737-100's and A300B2 but have long since ditched them for 777's and 737-500's and -800's. If you can have a more efficient fleet that makes more money why not upgrade. That being said I understand if your operating a lot of a certain type so replacing them can be a annoynce. I could also imagine wanting to keep a plane you really like in service because its your favourite or something