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.
Of course if we think about Forex currency trading market it is a 5.3 trillion dollar market it is that huge and all the trading assets are currencies but fiat once and like in share market instead of buying share in companies they buy currency pairs and there is no different in Bitcoin and the only change is it is a digital currency so there is no wrong in that people invest in Bitcoin market