Yeah, the argument is not intended to be “here’s the optimal contract/modification to complete the preferences”. The argument is roughly:
If preferences are strongly incomplete, then there exists at least one contract/modification which is a pareto-improvement.
Therefore, non-dominated strategies must not be strongly incomplete.
So, the argument in the post allows the possibility that the preferences will be completed in some other way which does even better.
Yeah, the argument is not intended to be “here’s the optimal contract/modification to complete the preferences”. The argument is roughly:
If preferences are strongly incomplete, then there exists at least one contract/modification which is a pareto-improvement.
Therefore, non-dominated strategies must not be strongly incomplete.
So, the argument in the post allows the possibility that the preferences will be completed in some other way which does even better.