Let's look at the mortgage debt crisis in the US through this same lens. It's just the US owing debt to itself, so you could just wipe it off the table with no consequences. And in large part what happened was a lot of people in the US walking away from their debt. Allowing their underwater mortgages to go unpaid, letting their houses be foreclosed. It turned out to not be particularly good for anyone involved.
Japan's debt to itself won't go any better if it is written off either. There are people, human beings, on both ends of that debt, snapping the cord will have real-world consequences, it's not just numbers on a balance sheet.