currency refers to a generally accepted form of
money, including coins and paper notes, which is issued by a government and circulated within an economy.
actually bitcoin is form of money, we can buy things by bitcoin, and it is issued by government and circulated within economy of a country like Japan. so according to the definition, bitcoin is a currency, even it has another function.
Yes, the property of investment opportunity will not last for ever. Because the demand of bitcoin will get saturated according to the world population regardless of its limited supply. But this will happen only after 200 years. So, as of now bitcoin will be serving us in both the functionalities.
Maybe in future itself, bitcoin will be identified as only currency as the price appreciations will not be in significant levels at any time like gold is doing right now.