I like this argument. But in this case I think there's another argument that doesn't rely on morality so much.

Your belief that the two theories in question will always make the same predictions is conditional on the box being perfectly sealed, and the universe continuing to expand forever. There's a small chance that these things are not true, and if that turns out to be the case, you may or may not expect to see the guy again, depending on what physical theory you believe in.

I think Matt Simpson is getting at this when he talks about counterfactual predictions.

You could counter this by inventing the physical theory that says that the guy in the box disappears, but if somehow you ever recover the box and open it, then the guy will reappear as if he had been there all along. But if we're going that far we may as well posit the theory that the Moon doesn't exist even though we can see it. The definition of existence should involve morality.