From whom are TUI going to source 737 Max replacements ? There aren't that many ACMI 150+ seat aircraft floating around from airlines that would be considered reliable enough for this. Leasing a worn out 30 year old A320 from a startup in a country in Europe that is sourcing aircraft from a boneyard and crewed with super-cheap labour is probably not ideal, unless TUI are prepared to deal with the bad
PR when it (most likely) all goes wrong in July (probably while French ATC are on strike, just to make thing more interesting !)