then US dollar is also an investment.
i'll tell you why. where i live the value of local currency is dropping so there are many who are now buying USD to stay ahead of the inflation that is going on. in fact they are investing in USD. and what they are doing matches the description you posted here, they will sell it in the future to get a huge profit.
but it doesn't make USD an investment. US dollar is still a currency no matter what you do with it.
the same thing is true about bitcoin. bitcoin is a currency by nature even if majority are using it as an investment.
Why are you saying that it doesn't make USD an investment, a lot of people in the world are doing what you've described - buying USD because their local currency is dropping, which can be called "hedging". Nothing is created to be an investment, investing happens spontaneously when people think that they can get profit. Bitcoin is and always be a currency from technical point of view, there shouldn't be any debates here, but economically it's most common use-case is investing for now. At some point in the future this will change, because volatility can't last forever, and at that point it will truly become currency in every sense.
But this debate is kinda pointless, because it doesn't matter how people define it, you can always use it however you want, the protocol doesn't care or even know about it.