How hard is it to build a Qassam rocket? Over 2000 were launched in 2008. I expect this type of drone would take the same level of engineering skills once the kinks were worked out.
As for the stealth factor. Let’s assume the drones are launched evenly throughout the 1000 mile diameter range, all at the same time. That’s a search area of 3M sq miles, or one drone every 3000 sq miles. Half way through the journey, with T-10 hours, all drones in the swarm could be evenly distributed in 785k sq miles, with one drone every 785 sq miles. An hour away, they’re all in a 7850 sq mile search area, or one drone every 7.8 sq miles. 5 minutes out, still a search area of 54 sq miles, and the drones are spaced out by over 1000 feet. 1 minute out, they’re all a bit less than a mile from the target, but up to 6000 feet up. An army of 10,000 soldiers armed with AR-15s couldn’t hit the drones at this point. 30 seconds out, the drones are 3000 feet up and still out of range. 10 seconds out and the drones are 1000 feet up, barely within range, but it doesn’t matter because they’re all on a terminal trajectory. Now you’re trying to shoot 10000 armed hand grenades falling from the sky. Good luck.
As for drone catcher nets? Let’s say half the drones are each armed with 1 pound of thermite, and ignite during a terminal trajectory from 1000 feet. What’s going to stop 5000 lbs of thermite followed by 5000 lbs of tnt?
These drones can climb to a few thousand feet, and weigh around 10 pounds. Can you visually identify such a drone versus an eagle? At night? When they’re 1000 feet away - 10 seconds from impact?
Can you hear them at 5000ft? 10,000ft? The same RC hobbiest who flew 1888 miles also set an altitude record of 26k feet.
Or they could fly at 50 feet above the ground, obscured by trees. Dismissed as your neighbor flying his RC plane, too low to be seen by radar, and looking like a cell phone in a car for all RF purposes.
Such a swarm goes from almost invisible to the explosive power of an F/A18 in seconds.
Besides the radar signature of a half pound engine, how else can they be detected? They have the same RF signature of a common cell phone.
I can 3D print amazing RC planes in 24 hours. Are they capable of 1888 mile range? No. That took an expert thousands of hours to perfect. But you can probably get an electric powered 3D printed plane capable of flying 50mph for an hour - today. Tomorrow maybe for two hours. In a decade, maybe 4.
Currently that number is extremely low. It will remain low until 3D printing is a complete zero-effort solution - which is going to take a very long time.
The reality is that this kind of open source terrorism is an incredibly unlikely threat. Numerically there are almost no terrorists, and the terrorists that do exist have almost no engineering skills.
Weaponised engineer terrorists would be a scary thing, but they don't exist in non-trivial numbers. And unless something very bad happens, that's unlikely to change.
As for the drone scenario - it's not exactly stealthy. At 50mph there's roughly 20 hours of warning - plenty of time to put up drone catcher nets, or even launch a suicide drone swarm.
It's not that drone attacks aren't a threat, but they're going to need to be more robust than anything a DIY off-the-server design can produce for some time to come.