Your knowledge is impressive but my guess is that they are still using above ground infrastructure in rural areas because of the high cost of underground infrastructure. Underground lines are usually reserved for cities where they can serve a large number of people and their cost is warranted. But you are probably right about rural areas using off-grid tech, like satellite and solar. But things like home batteries are quite expensive. So those are added costs which drive up the expense of living off grid. But my understanding is that Japan is not very welcoming to foreigners. But you could buy a very similar house in a rural area which caters to foreigners, like in Portugal, Spain, Italy, Thailand, etc… and and do the same thing with fewer bureaucratic hassles.