Some good points raised above -
TUI CEO mentioned that if the MAX is not back by the end of APR (We know it will not be) then their financial contingency allows to remove the MAX from their S20 program.
It is extremely hard to try to introduce, or re-introduce a new, or an amended type that will require intense pilot training/SIM time, and the various Crew and Ops manuals to be rewritten) in the midst of a summer Charter flight series as big as TUI's.
I think TUI now have pretty much taken as read that the MAX will be out of the their Summer 2020 flying program - My Tuppence.