If you took out a dam in Montana the following chain of dam failures would cut the United States in half all the way to the Gulf of Mexico and destroy US agricultural output. The US produces 40-50% of the world Soybean and Corn supply. Long term you're probably talking billions of deaths due to food shortages.
Dams are dangerous beasts. The KMT breached some dikes in the 1930s for the purpose of environmental warfare, and in the process killed half a million peasants and displaced millions more.
I'm not sure it would be. I don't think we've considered all that can happen with a sophisticated worm.
The problem with leveraging nukes is MAD. With worms, you can do a lot of damage without even knowing who did it. Think the Anthrax attacks in 2001 x 1000.
With worms, you can do a bunch of damage over a long period of time without getting discovered. What's the US going to do, start a nuclear war over it? No, see MAD above.
I mean if a worm could figure out how to stop shipping (say simultaneously disable control / start systems of vehicles or gas pumps), people will start to starve after a few days, then probably total chaos will happen leading to a bunch of deaths. That's just a single scenario.
How about if a worm took control of all the air traffic towers simultaneously and change the information so that controllers would start crashing planes everywhere.
I know nuclear war has been played out on tv and in movies for the last 70 years or so, but I don't see an all out nuclear war between two states lobbing hundreds of warheads at each other ever happening. At least not intentionally. Any type of nuclear detonation would either be accidental, or very isolated.
Mosul dam in Iraq was in serious trouble and some argued that it might collapse after the second Iraq war. If it's ever breached the disaster could kill as many as 1.5 million people living in the city below and displace a further 5 million. It's not beyond the realms of possibility that a Stuxnet aimed at the dam's control systems could kill more people than at attack with nuclear or chemical weapons.