The Department of Defense has a much larger purview than propping up the value of currency. Cryptocurrency doesn't fight terrorism (at best, it's agnostic to terrorism, at worst, it aids it). It won't fight back against the next Pearl Harbor or 9/11. And if your reasoning reduces everything about society to the economic output/currency value of that society, it might be time to get some fresh air.

Additionally, regardless of whether the DoD and all governments of all nations spend more energy or warm the planet faster than global cryptocurrency, cryptocurrency is superfluous—the global financial system was working without it, and could work without it again. Everything cryptocurrency "fixes" about the global financial system could be fixed more surgically, and without a system that exerts so much stress on the planet.