> It’s very easy to take down a drone.

For sure. Laserpointer to blind it, jam gps to confuse it...

> And in order to tase or spray someone it has to get really low.

For tasing, sure. And for spraying an individual in the face. But if we're talking swarm-based crowd control, there might be other options. Anything bad that drops down (say 1000 minature cs-cannisters -- effective even if a few are "thrown back", other gas that is heavier than air, perhaps created by blending two components "on board" the drone, perhaps forms of acid or irritants liquids that are arezol'ed with pressure... audio-attacks...

In fact, swarm based drones might even be hard to take out, if they could rely on low-bandwidth, near-field comms, such as IR, lasers, (directional) ultrasound or whatnot. Not even sure if there's much current research in combining directional ultrasound with the type of advanced coding that is used for high-bandwidth radio short-wave/microwave comms.

  >  in order to tase or spray someone it has to get really low. 
  > These things are very slow and not that agile.

For now.

And it will become hard to even see/notice them as they become smaller and quieter. What then? Shoot down a fly?

You’re talking science fiction now. If it becomes so small as a fly how on earth would it be energy sufficient? With a battery? And that battery will hold enough energy not only to keep it flying but also to provide for a taser that could incapacitate an individual? And at that size it should also carry some communication infrastructure to receive instructions from a command center, a gps, and a cpu capable of running face recognition or even detect movement patterns because otherwise you can’t have efficient crowd control. Sure, in a couple of generations we’ll get there, but not in the immediate future.

And 640K should be enough RAM for anybody. Betting against technology is a losing game.

My nephew has a palm-sized copter with remote that flies for 10 minutes, right now, got it for XMas.

There have already been dragonfly-sized gadgets made by graduate students. Check out youtube.

RFID tags are all the computing power you mention, already on your high-end product you bought from WalMart.

The future is clear. Maybe a one-use metal-air battery on a chip-bot? If I can say it, its conceivable.