Also the idea might work if the whole world thought the plan had crashed. But now that suspicion has changed to it being hijacked and flown somewhere then that kind of minimizes it's usefulness. Everyone is looking for it now, and are on the lookout for what it might be used for.
Logistically and strategically, it would have been simpler to obtain a surplus airliner, move it to a remote location near the target. Refurbish it, load it with explosives, then fly it at the target. They would have the advantage of time and would not have to penetrate airport and airline security. Such an operation would also show that airport security is immaterial to protecting the public.
But then what do I know.