The whole 'massive conspiracies can't exist because they are impossible to maintain' is a totally fallacious straw man argument. How is it people can say that sort of thing while simultaneously believing a small conspiracy of cave dwelling Arabs managed to orchestrate the whole thing?.
But no-one's claiming that at all. Everyone know's bin Laden's the scion of a hugely wealthy, well-connected oil family (indeed, this is the subject of conspiracy theories in its own right) and has an enormous personal fortune.
And in any case, the logistics of committing 9/11, considerable as they may have have been, pale in comparison to the degree of organisation and secrecy that the US government would have had to maintain in order to carry out the attack and get away with it. I think it's also pretty telling that not a single person, anonymously or otherwise, has acted as a whistle-blower for this supposed conspiracy. Neither, as far as I'm aware, has any attempt been made to stop these fearless troofers who risk everything to spread the word by prining tee-shirts and putting videos up on YouTube.
Bin Laden is very much the missing piece from 9/11 CTs. After all, think about it from his POV: if he'd either been allowed to commit the attack by a complicit CIA/FBI, or if he'd actually had nothing to do with it but had had the blame pinned on him, why would he play along with it and accept responsibility? Surely he'd have said "Look, America, at what your own government does to you for its own selfish ends!" and then sat back and watched as the country destroyed itself in violent revolution? The only way around this is to claim that bin Laden is actually in on the conspiracy himself, that he is somehow the CIA's stooge, or paid-off fall man. Which is another problem with CTs in general: there is no limit to how high up the conspiracy goes, how general and all-encompassing 'They' are.