I would say this is not about terrorism per se, but rather that, as you mentioned, we rely on an ever more complex and fragile infrastructure.

In medieval time, the only network you could seriously sabotage was probably water, by poisoning a well or a stream. In the XIXth century, it was the railroad. In the later XXth, it would have been the electricity grid, and today, it is increasingly the information network.

And even if we rely on additional layers, you can still sabotage the more primitive network, e.g. the Mosul Dam in Irak.