Pop takes no prisoners.
If you are not toned, fit, alert, rehearsed to the hilt, and ready to do it right, then don;t step up to the plate.
Britney's advisors should have told her that she simply is not ready to return to this level of performance, especially in such a high-profile setting where her poor rehearsal levels and apparent unwillingness to look anything more than a pale karaoke imitation of her former self should have been noted, and stopped.
Yes Britney has had her troubles, and two children, but pop doesn;t care, it has the attention span of a gnat, and the loyalty of a preying mantis, and it will, and has discarded its falledn as it always has, and always will.
Do the decent thing Britney, go gracefully.