"If "boots on the ground" can't shelter behind tanks and inside APCs to get from point A to point B, then all our doctrines and tactics have to be re-examined. It's not clear to me where that would lead."

That's exactly what I'm saying. The only defense I can think of that might shield people (military or civilian) against the kind of armed drone attacks we will see in the near future is electronic warfare e.g. spoofing, electromagnetic pulse, hacking, etc.

There's other possibilities like lasers with a high rate of fire (which our mlitary is working on). I sincerely doubt we could fight a drone swarm with another drone swarm - the targeting would be incredibly problematic. One might think that a more accurate version of the Navy's CIWS (close-in weapons system, a 20mm Vuican cannon) might serve, but against a swarm...no way.

The only real defense is something that can disable the ability of the drones to communicate with each other or with whoever is in command of it. Even then, if they are (1) able to truly operate autonomously, and (2) are hardened against EMP, I don't see how they can be stopped short of an effective point defense using rapid-fire lasers.