re: If the Borderland states left what would happen to the rest of the US

Do you know that all US military assets were purchased by and currently owned by the United States Government?

This government is not going to leave this capability to another, plain and simple.

The new nation/s are free to fund and develop their own tech, they may surpass the US at some point due to advantages you mention. This makes me uneasy that this would cause serious tensions down the road.

I'm not picking sides, I'm just answering your question and straight up telling you no gov gives up its advantages without a fight, they especially would not allow the potential threats this could cause them. It is not how the world works

And if you think the US needs them more than they need the US this only hurts your argument that the US would give up these military advantages.

They both need each other to be what the US currently is, both would be offered what they are missing in the split by opposing world powers. They US gov knows this

There are endless examples throughout history of major conflicts that seemed easily avoidable. That will be no different in the future, with this being one of them