I don't think Bitcoin will ever be accepted as mainstream currency of a country because no govt. ever want their citizens to shift to any alternative currency whose transaction can't be traced. It can lead to serious national problems like tax evasion, money laundering, buying/selling of illegal stuff etc.
On the contrary if governments sees the potential of a crypto currency and just invest on how to compensate the losses that they will have from that shift then it will be a win-win situation. Japan is known as a country for innovation and the fact that they are supporting the use of bitcoin in their country means that they are seeing it's potential as a future currency and most likely they are already studying on how to atleast have some regulations for it without compromising it's primary goal.