Anti-Truthers themselves seem to rely heavily on the implausibility argument when debating the 911 controversy. The alternative conspiracy narrative in regard to 911 is simply impossible, or implausible, because it does not make moral and technical (to them) sense.
A strange assumption, since to believe this they must also believe that, as stated through the mainline conspiracy narrative, we are living in a "post-911" world in which everything has changed.
In this case why base your assumptions today on assumptions you had before the 911 attacks (ie for example such a covert operation is impossible)? Hasn't "everything" changed?
No, and it's important not to confuse hyperbole with fact. The statement that "in the post-9/11 world, everything has changed" is quite trivially false as an exact statement of fact, and is therefore an example of hyperbole; taking it as justification for the belief that things technically impossible before 9/11 are now technically possible, therefore, is a classic example of the fallacy of equivocation. The applicability of Newton's laws to non-relativistic events, for example, has
not changed since 9/11, nor have the laws of causality, the rules of logic and the law of conservation of energy. Propositions refuted by the proper applications of those laws are therefore no less refuted because of an exaggerated, and fundamentally political, statement that "everything has changed" since 9/11.
And, in fact, none of the assumptions have changed that much either. Before 9/11, there was no presumption that the hijacking of airliners was impossible, nor that suicide attaacks could be made by crashing aircraft under control into specific targets. Both these acts were in fact well-known, the former from numerous hijackings carried out as hostage-taking operations, and the latter from Kamikaze operations in the Second World War. Nor was there any particular assumption that these could not be combined; it was simply a scenario that few people other than the planners of the attacks had seriously considered.
Dave
ETA: Nor is it incumbent on anyone to believe every aspect of a single specific narrative. You'll find many different shades of opinion on the level of negligence on the part of the Bush administration, and on the level of honesty with which they conducted themselves after the fact in planning and popularising the wars in Afghanistan and Iraq. Your belief that "they must also believe [...] the mainline conspiracy narrative" is nothing more than a statement of your personal prejudice.