I only saw one man in one tank. He might not even be dead.
Angier doesn't want the machine used again because people might catch on that it's just a Tesla coil. He doesn't want Cutter to see him set up one man to be murdered and another to be framed, especially because he needs Cutter to testify later.
Just one man in the tank? Let's for a split second accept your theory that the machine doesn't work..who is that 'one man' in the tank then? Even if u argue that Angier is somehow performing the trick every day on stage (God knows how) we see
1. that one Angier dies in the tank on the last show
2. that another Angier survives under an assumed name
3. even when he gets killed by Borden, there is still 'one man' in the tank
Just explain these three cases
Tesla understands obsession, and he knows that Angier will use it to trick another Magician. Hence the warning.
Tesla , a scientist, would be the last person to worry about Angier 'tricking' another magician..and that would be the last reason for the ominous warning.