You're making it more complicated than it is. A million dollars worth of bombs that are dropped on some foreign countries is not the same as a million dollars worth of road improvements or schools or tech investment. There's no return on the bombs, they don't make your country richer. There's return on the other investments.
USA already has adequate roads and world class schools, they already lead the world when it comes to innovation.

I don't think I'm making it complicated enough, Geopolitics is complex as fuck. What I can say with reasonable confidence is that those in power in the USA, and I'm not talking about presidents, are not stupid. They are intelligent sociopaths or maybe even psychopaths that understand the complexities a great deal better than I do. With that said, why do they prefer to invest in war than roads and tech? Corruption and personal greed? Maybe. Or economic self preservation? Also maybe. What I do know is that lots of countries have large reserves of dollars, which means that if the dollar crashes, so do lots of economies around the world. That's no accident. USA have created this situation to ensure that any attack on the dollar is mutually disadvantageous.

I'm not an expert either but I know enough about history that countries' relative power is not a static thing. I think that's pretty obvious. Its also a historical pattern that the most powerful country overspends and overextends itself militarily until it's no longer the most powerful country. That's going to happen to the US eventually (at least the way it's going now). And the most likely country that's on a trajectory to replace it at the top is China.
I'm probably in agreement with most of this. I just don't see China replacing them, not the way they're going. Their ambitions seem very short term. Maybe I'm not giving them enough credit, maybe these ghost cities are a well considered plan for the future, but it seems to me that it's just creating growth for the sake of it.