Hotels are already cheap in Japan ($60 a night will get you quite a nice business hotel with hot spring in most places I've been) and Airbnb is subject to local restrictions (federal law passed recently allows local governments to place restrictions on Airbnb - you have to get a permission letter from your city Hall to be allowed to list on airbnb - and they can deny you for any reason eg neighbour complaints). Personal use would be a possibility for sure.
My point is that there are many young Vietnamese hoteliers who would love to move to Japan and operate some of these abandoned houses as hotels. They will have to do all the renovations, cleaning etc. themselves, but they will earn much more money.
SE Asia is a completely different economy though. Japan is a first world country, and the third largest economy in the world. Its not going to cost the same as someplace in SEA.
In addition to what everyone else said about it not being a useful comparison to look at SE Asia, I also wasn't looking at the cheapest options. Capsule hotels would generally be available for half that ($30) and sleeping in a manga cafe would be $10-$20 depending
