You're right. BUT to kill 90% of us they'll need to get a nuke up high enough to cause the emp effect over a wide area. One nuke 125 miles above Nebraska will wipe out the grid and un-hardened computers chips.
Most of us would die within the first three months.
That said, of course you're right - they'll bang for their buck if they walk a nuke across the Mexican border or do the yacht thing. But that damage is regional and we'll be able to totally destroy North Korea - and survive. We would not survive as a first world country if our grid was taken out...
Because we're too stupid to rebuild?