The net upside is maintaining the market that allows for a trade deficit in the first place.
The problem is really that the two aren't directly linked. If you operate at a trade deficit, you don't need to manufacture at home. But if you can't manufacture at home, you can't build war machines efficiently, so you can't be the world police and ensure the market needed to operate at a trade deficit.
By defunding the military you're pulling the rug out from under yourself in this sense. I would still agree that it's massively inefficient and needs to be fixed, but the working US-World system would still invest a lot of funding into the military.
Unfortunately, at some point the people who are trading products for dollar bills will catch on that there's nothing that they can buy with the dollar bills that are worth the value of the products they sent across the ocean. When that happens, people will stop accepting the dollar bills as payment... but by that point, you've hollowed out your manufacturing to such a degree that you can't ever start it back up. So you can't trade for those products (no products of your own to trade), you can't purchase them (your own currency is worthless), You can't purchase them with other nations' currencies (nothing to sell for that), and you can't make the products yourself.
But "soft power". We traded long term prosperity for the short term ability to meddle in other people's countries and lives.
You're leaving out a large part of the equation, which is that countries can trade with each other in the reserve currency, which becomes a neutral medium of exchange with respect to the parties. The value of holding US treasuries currently is that everyone will accept them. By having the world's reserve currency, the US becomes the owner of the world marketplace. It's not just about trade with the US. I export to the US, get USD which I convert to US treasuries, and now I have near frictionless trades with everyone else in the world.
One huge impact of this over the last 50 years is that the world is effectively lending the US liquidity at the lowest possible interest rate, which the US then extends as credit to American businesses, universities, etc., fueling a boom in industry, commerce and academia.
Another big impact is that holders of US treasuries are strongly incentivized to support and protect America, because as America falls, so does the value of those holdings. Before China declares war on the US, it looks at its huge pile of treasuries that will become worthless at the first shot, and says "maybe not". It also allows the US to affect other countries through its monetary policy--another incentive for others to play nice with the US.
It's kind of boggling how little this is understood, how much short and long-term prosperity the US has created for itself by doing this.
The problem is really that the two aren't directly linked. If you operate at a trade deficit, you don't need to manufacture at home. But if you can't manufacture at home, you can't build war machines efficiently, so you can't be the world police and ensure the market needed to operate at a trade deficit.
By defunding the military you're pulling the rug out from under yourself in this sense. I would still agree that it's massively inefficient and needs to be fixed, but the working US-World system would still invest a lot of funding into the military.