Except the machines will only fight each other as a defensive countermeasure. The goal will be extracting a political surrender by the government as always by which means terrorizing the populace and inflicting mass misery is the most effective means of pressuring the government. The armies purpose is to prevent that from happening, and as the armies disappear and it’s just metal on metal, the civilians become the ultimate target of the machines and much sharper in focus by the war planners.

Also it becomes easier to do things like sneak a kill bot drone into a city and release it as the ultimate asymmetric warfare aka terrorism. Your terror attacks no longer scale with your ability to train and insert fighters willing to die.

The truth of the matter is we already have the technology at the DIY level to do this, and it’s just a matter of time before AI kill bots become the standard of war.