I think that's apples to oranges - I really think there is a difference in loosing military control of an empire versus losing economic dominance. The US isn't going to cut spending on it's military substantially because china makes a better cellphone than Apple. Apple will continue to compete, and consumers will enjoy better phones at lower cost.
But if China makes a wide range of products better than the US then presumably they'd be able to afford to spend more on their military than the US does?
I'd argue they already do, America just captures a lot of the value of their labor back home. I don't see this arrangement changing anytime soon.
I'd also argue that, while I see your point about less tax revenue, military spending seems to march to the beat of it's own drum regardless of US's coffers. I don't know if that's sustainable to be that decoupled, but it seems like it is right now.
