>drive-by type attacks would be very concerning

not only that. One insecure system in the area could be infected and then controlled to attack other networks in the area. With a high density of IoT devices you could have malware spreading wirelessly device-to-device. Also with a wide IoT adoption with such devices being used by public/utilities companies (smart street lamps, leakage monitoring in pipes, smart electric grid etc) you don't even need a densely populated area to have that problem.

>I am more concerned about the bigger threat, global attacks.

I once read something among the lines that IT security needs its own Pearl Harbor event. I can neither quote it exactly or attribute it to any source