Many many moons ago (around 2009 I think) Nokia _did_ have maps for all of Japan available for download. They didn't have turn by turn navigation, but they did have all the streets. I used them on my E90 when I went over there and it worked a treat. Even better, if you wanted to you could use Google Maps on it to find a specific location, save it as a landmark in google maps and it would show up in Nokia's maps as well.
When they upgraded to version 3 of the maps a few years later with the new firmware, the latest maps for Japan only showed the highways and wouldn't work with the old V2 version of the maps. When I went there again late last year I had to downgrade my E90 (yes, this is still my current mobile phone :-)) to an older version of the firmware and put the maps back on again from an old backup I did back in 2009. Keep in mind that this phone is still running Symbian... try finding support for that now! Luckily I kept the install files for Google Maps as well as I don't think you can get the Symbian version of that anymore either, although it still works with current Google Earth data quite happily.
I don't see any reason why Nokia couldn't at least make this old data available and bring it up to something usable with their current mapping programs. They clearly had it, then got rid of it, and don't offer an alternative for offline mapping, or even their browser based mapping software.