But that can’t tell you that it didn’t evolve for the same reason that such a calculation can’t tell you that a particular set of cards can’t be drawn. Merely saying that from the perspective of the ancestral/prior state some particular outcome looks unlikely can’t tell you that the outcome you do have wasn’t, in fact, produced. From the perspective of your great grandparents 10 generations back your existence would seem miraculously remote too. And yet here you are.
So I just have to ask, what is the value or significance of doing such a calculation with respect to evolution? They give us no reason to think that any extant structure didn’t evolve.
I must conclude you have no rational basis for thinking any known biological structure or system was not produced by evolution. There is no reason to think intermediates in flagellum evolution would be nonfunctional(they’d just have functions other than cellular motility), and the a priori probabilites you calculate can’t rationally support the conclusion that X didn’t evolve in the same way they can’t support the conclusion that you aren’t the product of 10 generations of ancestors.