IT depends how it compares with capital gain and income tax in the country in general, and how they handle losses. Also depends when they calculate capital gain (at the moment of conversion to JPY, or each conversion between different cryptos).
55% on insane free money this year would have been just fine. it won't be fine when (if) crypto ever stabilizes. there again, it's early days, and governments will need some time to tune things.
the clearer the regulations the better.
Yep! We are standing under the new adoption of crypto from different countries or even union of them as they are sure for now that they just can`t simly ban Bitcoin.
And in the first stage of this adoption every country would want to benefit more. But then compettion will make this fines, fees, taxes to belower as only the wisest will win the more)
As to the circulating supply of the BTC, I think that in the near future (2-3 years) it wouldn`t cause the change of the price! And, by the way, nobody exactly knows what supply is burnt by loosing the keys to the wallets.