I think you vastly over estimate the 'criminal underworld'. The DOD might be able to make miniature lethal drones using facial recognition software in 20 years. But, probably not. The idea that criminals will start mass producing such things cheaply is silly in your lifetime is just shy if ridiculous. Talking about building stuff is easy, but actually building stuff is hard.
PS: Problem #1 people don't look up that often. Problem #2 there are a lot of people walking around out there. And it just get's worse from there.
Well, I'd love to be wrong about this and hope that you are right. What worries me is that key parts of the technology, like the processor, sensors and software might be available cheaply due to their civilian uses. Manufacturing ability could well be scarce, but these weapons could pass through illicit distribution networks which today distribute machine guns and rocket launchers. I didn't fully understand your postscript, but regarding visibilty, I admit I dont know much here - for instance, how small of drone would be realistically possible for a non-state producer. But I dont see a clear knockdown argument, given that drones are used effectively today. Again, I hope you are right and that there are more technical obstacles out there.
It is not about making it yourself from scratch and doing research and experiments, I think it is about buying the parts from China and assembling.
Say IED builders in Afghanistan, probably do not have PhDs in physics and chemistry but they can build effective shaped charges to pierce thick tank armor.
So at some point you could just order parts from some place. Assemble, then upload the picture of face it needs to recognize, approximative coordinates and press the "take-off" button.
I don't know, I think this sounds disturbingly doable in the near future. Both your problems are essentially "opportunities to strike are few." But the point of drones is that they're cheaper than humans. If you can afford to tail someone with 2 goons, maybe you can afford 20 drones all along their typical route. And the drones can recognize license plates and other non-facial things to help them hone in.
As for facial recognition software, there's probably already an Android app for that. Sure, there's a lot of work involved - it has to recognize the person, it has to drive around autonomously, it has to aim and fire. But most of those may be pluggable components of hardware and software soon, coming from perfectly innocent projects.
I think drones may well be a big problem in the near future.
There's money in that. There's less (steady) money in assassinating the president. In fact, I imagine assassinating a president creates more problems than it prevents.
PS: Problem #1 people don't look up that often. Problem #2 there are a lot of people walking around out there. And it just get's worse from there.