Leasing in an aircraft does not say 'long term commitment', it's another way of saying that NI holidaymakers aren't worth as much to TUI as other travellers
Sorry, that's just nonsense. The seasonal cross-Atlantic exchange leasing between TUI and Sunwing is a sensible approach to managing the two airlines' respective peaks ( winter snowbirds to Florida, summer sunseekers to Med beaches ). Transat and ASL France do the same.
The 'value' of the NI holidaymakers is such that TUI see value in sustaining that leasing, rather than just focusing on the subset of services possible using the core 'blue' fleet.