Ok yes, in a single transaction you can prove to everyone else that the net exchange is zero, but how do you prove that you have enough money to send to them? T

Ok yes, in a single transaction you can prove to everyone else that the net exchange is zero, but how do you prove that you have enough money to send to them? That's global state that depends on all past transactions, even if they're hidden. Include more ZKPs for every transaction ever associated with that address? You have to prove that 1. you received enough to cover it and 2. you haven't spent it already. Just slapping some ZKP on top of bitcoin is not enough to make it magically private. It needs deeper integration to the model than that.