If we're making an edit that's designed to be watched in chronological story order, I'd like TPM to be able to stand alone, as though it were actually the first one made, the way ANH does. By "first chapter", I mean it would feel too incomplete.

There's no rule that says it has to be the lightest movie, but as it stands it is, and giving it a downer ending doesn't fit the mood of the rest of the picture. You'd have to make a LOT of (otherwise unnecessary) changes elsewhere to make it fit. The Phantom Edit and Balance of the Force are both darker than TPM, but still lighter than any of the other movies.