To date, there are literally dozens of groups of hobbyists who compete with FPV vehicles (both ground and air) to deliver large pyrotechnical devices to "goals", from over 4 km away. It's not even expensive or difficult...it is off the shelf and an amazon.com click away.
To date, there are at least a dozen people who have equipped a vehicle with FPV transceivers and the simple servos required to navigate through actual city streets while miles away themselves. Latency is not the issue that some people who haven't actually tried it might argue. To be fair, the videos I've witnessed were done at night with minimal traffic present.
These things are relatively cheap, not very difficult, and are completely available to anyone with some time and motivation.
This has been the case for a very, very long time. This is no game changer.
The game changer would be the sudden appearance of legions of people with a little money and a lot of motivation to use these things for nefarious purposes.
So, the question is this:
Why isn't this happening all the time?
1) Either people just don't know how easy, accessible, and cheap these things are, or
2) All the luggage searches, border security, and spying on private citizens is batting 100% for effectiveness in preventing the legions of terrierist attacks that must be attempted every day, or
3) These nefarious people simply don't exist in any number great enough to worry about.
Hypothesis (1) is naive and silly. These ideas are the first thing to occur to any casual 14 year old pyromaniac nerd. They aren't the last to occur to occur to a determined, capable theoretical "terrierist".
Hypothesis (2) is what comprises the confidence game we willingly pay trillions to every year.
We live in a world where hypothesis (2) is the only likely scenario, and should be considered "theory" by now given the ridiculousness of (1) and (2).