It is unsustainable long-term, but it's a reasonable model to follow. Build today while the cost is cheaper, create a larger general economy and then coast for a while. Japan also spent decades boosting GDP with huge infrastructure projects. They can't sustain them anymore and in some areas are starting to dismantle some of the excess but the Japan of today is much nicer place to have a flat economy than the Japan of the 40s or 50s.