The 757's were all supposed to be gone by this winter's end, that was the plan for 757 retirement, but as we know of the 12 (3 have just gone) 9 remain and
afaik their disposal has been postponed (indef?)
I don't think TUI have been inept in anyway since March when the MAX was grounded (They had 7 in service that week, with 3 more due, plus loads due for next summer, plus the -10 series too)
I think they handled the whole ACMI lease-in for their S2019 program immensely well considering the grounding occurred just weeks before the Summer season, but at a huge cost to them - The forecast was 600m euros.
The Sunwing lease arrangement of their fleet returning to Canada for the winter is a historic one as you know (Like AE did the same with Air Florida, and Air2000 with Canada3000)
But Sunwing have their own MAX grounded too, so do we assume the same number of 737NG's come back to TUI for Summer 2020?
Some that are going back to Canada now have retained G- reg's.
As it seems the RTS of the MAX rolls over to at least Q2 2020 TUI can now decide what fleet arrangements are needed to cover the groundings.
TUI have announced massive expansions and added destinations (2m more seats) so it needs planes...