That formerly high savings rate was the only thing that enabled the Japanese government to borrow as much debt as they did.
Now that the Japanese are no longer able to save enough money, the government can't continue to borrow from that source, and accordingly the govt. has been forced to turn to the last measure available: currency debasement.
The thing is, though: the Japanese government isn't trying to debase the Yen because of their high debt. The reason they're trying to debase the Yen is to encourage growth via exports. Somebody has still been buying plenty of Japanese government debt all those last years. It would be interesting to know who.
While I appreciate your comment, why don't we think outside the box a bit? Isn't it good when people save a lot of money? As long as the savings are fairly well distributed, this means people become more economically independent. In other words, they have more freedom.
I believe this is something that governments should support[0] rather than just taxing the savings away. Granted, however, that an unequal distribution of those savings can become problematic. Perhaps a compromise can be reached with a wealth tax that is indexed to median wealth?
[0] Which does mean that demand gap needs to be plugged somehow; the most straightforward way to do so is via a government deficit.
So, one proposal: institute a small yearly wealth tax.
Avoids a lot of the problems with inflation-based approaches, and doesn't penalize people nearly as much for having liquid assets.