The biggest part of that economic war would be letting everyone know the grid has been compromised, not the actual outage, there's no advantage to keeping it secret.
I'd argue that taking out the power in three major economic centers at the same time would be the opposite of keeping it secret. But I completely agree with your reasoning.
But the downside to making it public is you'll galvanize the victim nation to start taking computer security more seriously, and your window of opportunity will start to close. It would make sense to try to "boil the frog" and gradually and slowly increase the frequency and severity so that by the time you "find" the threshold that snaps the victim nation out of their slumber, you've gotten away with a lot of damage cumulatively.