I would agree that your scenario would be an exploit. If that were possible I would have no hope of proving the scheme secure because it would manifestly be insecure. The reason it can be insecure in this case is that the utility functions don't satisfy the guarantees I wanted; I need human generosity now to be more valuable than world domination later. Maybe you don't believe that is possible, which is fair.

Here are some utility functions that would work. Producing the first {paperclip, staple, bucket, spoon...} as soon as possible. Producing a {paperclip, staple, bucket, spoon...} before the first {antipaperclip, antistaple, antibucket, antispoon...}, etc.