LeSocial> Some processes are called irreversible. Remember the “you can’t unscramble eggs”? No one knows for sure what exactly happened to the skies, perhaps some kind of chemical reaction, and some say it’s nanites (I don’t think so). It’s shown in Animatrix Second Renaissance, but we’re not told the details. But if it was an irreversible proces, it’s not enough to just stick “some other chemicals” up there in the atmosphere. It would take a massive amount of work, time and energy to restore and clean the skies.
(btw: No nuclear explosion would send dust up in the atmosphere for millions of years. And a lot of nuclear explosions would’ve wiped humanity out as well. )
The machines are not ONLY using human body heat. It’s combined with a KIND of fusion. It’s the movie premise that THIS is more efficient for the machines than attempting to restore the skies.