The WTC (which was also an office complex) suffered two direct hits. Less than 10% were killed.
BTW, the Pentagon is hardly desolate ~ it dominates a vast space because it's only a few stories tall.
Remember, they said they wanted to destroy the WTC. And they did, even though nobody would have said they could. All they had to do was to get lucky, which they could do again.
And I still don't understand why you are attempting to minimize the amount of damage they could do. Does that make their behavior better in some way? The fact remains that they want to kill as many people as possible. Even if they wanted to kill just one (such as Theo Van Gogh), that's not a happy thought. But they really do want to kill as many of us as possible, and maybe while you might feel more secure by scoffing at their destructive capabilities, the fact is, they want to do it. And they might well get away with it, particularly if more people have your dismissive attitude.