There's no reason to believe military spending couldn't be reduced in favor of research spending.... why not just put that money into non-military research anyway?
It would improve our country to do that, but I think I can answer the question of what some of the challenges are that we'd face to do it.
The idea that military spending produces civilian benefits as spin-off technology doesn't really enter the calculus for military budgets.
In the case of this plasma shield, it won't be marketed to civilians in any workable form (in my opinion), because it would confer too great a tactical advantage. There won't be a dime made from the patent described in the OP in civilian markets. Therefore, there's no way to even calculate what sort of long-term macroeconomic utility might come from the tech, say 20-50 years down the road. Hence,
none is expected when the budget is allocated.
The military budget is therefore not made with anything in mind except military utility or potential military utility. That's what's being bought with the money.
And herein lies the point. In order to make the reduction in military spending, one must confront the argument that military potential will be reduced by that. In my opinion, that's why the budget for the military tends to stay latched at high levels, because it's generally political suicide to imply that we should be less militarily prepared. There would be no concept of simply transferring some fixed thing (i.e., research) from one rubric to another. Instead, it would be more like reducing military preparedness, and in exchange having a more vibrant economy in the civilian marketplace.
You probably already know this, but it's worth spelling out in a discussion about it.