If we don't call it an investment then what is it? If I trade in forex and earned some profits because the price of the currency that I bought went up against the dollar, isn't it that we can call it an investment because the money I spent in buying that currency can increase or decrease after a short period of time.
There is no other sentence to describe bitcoin other than investment. after all there is no problem for that, many people put their money in bitcoin and hope their money can grow in bitcoin, if you look at the meaning and definition of investment you can certainly call it an investment. there is nothing to worry about, I think it's clear that bitcoin is an investment