So long as Flight 77 hit the Pentagon, one can imagine a rather small group of conspirators. With a bit of creativity, you can even get it down to one primary orchestrator of the whole 9/11 tragedy, who would be the only one that knows the truth about it.
But, in order to make a 757 vanish into the ether, you require a sprawling conspiracy. Who are all these people that not a single one has leaked? I don't even mean go to the media. I mean, there is not even the slightest rumor of an alternative fate for Flight 77.
Moreover, if this conspiracy is willing to crash the hijacked planes into the WTC, then why on earth not crash Flight 77 into the Pentagon? I mean, you've obviously got the plane under your control. Why go to the trouble of firing a missile or flying in yet another plane, or whatever, if you've got a perfectly good plane to crash into the Pentagon?
Think about it. You're planning to pull off this evil deed. You think to yourself, I will hijack planes and crash them into the WTC, the Pentagon, and the White House/Capital (or wherever the 4th was going), why would you then say to yourself: "Oh, but let's just make it look like a plane hit the Pentagon. Instead, we'll strike the Pentagon this way and spirit the plane off to wherever."
It just makes no sense whatsoever. Any way you dice it, there is no rational reason to just make it look as if Flight 77 hit the Pentagon.
Unless..... unless the passengers of Flight 77 were in on it too!!!