[wegs]

Precisely what you said: Power grid has security. Compromising 90% of connected devices with a mainstream OS would, at once:

(1) Tank the economy

(2) Likely, be doable with resources totaling in the single-digit million dollars

Modern wars are largely about industrial capacity.

But if it is equally easy to compromise the grid, why not do both?

This may sound like a movie plot, but so did the invasion of Poland at the beginning of WWII, the attack on Pearl Harbor, nuclear bombs on Japan, the rape of Nanjing, or many other actual events which Actually Did Happen.

We tend to underestimate the impact of rare events: our brains are conditioned to discount anything which happens once or less than once per lifetime. That's likely why humanity will kill itself at some point.