In my opinion the solution is for Japan to default on their debt obligations. They're already aggressively doing that in the classic stealth way, with currency debasement. The problem is, that approach is going to take decades, which ensures a non-stop erosion over that time. It would be drastically better to do it all at once, and to then get on with rebuilding while they have a larger working population. And while they're at it, reform the government that got them there to begin with so it doesn't happen again. There's no scenario where they can ever pay back their national debt, and payments on it are consuming a very large portion of their budget. If they take the long road, it's going to be a really bad 20 years at least, as their current problems are going to get far worse.