Why don't you re-examine your assumptions about how buildings should collapse under the conditions these were subject to? It's seems like an awfully small minority of architects and engineers are agreeing with you.

The Pentagon never had problems with perpetual war before this happened so that doesn't really seem like a realistic motive here. You don't need an excuse to do what you are already making excuses successfully to do. A conspiracy on this level is far more elaborate than what is necessary so why go to all the risk and trouble?

Sure, there are evil people in the world but enough of them on the boards of defense contractors to conspire to murder this many men, women and children? Anything is possible but this hardly seems like the most likely scenario.