In an economic sense,
An INVESTMENT is the purchase of goods that are not consumed today but are used in the future to create wealth.
In finance
An INVESTMENT is a monetary asset purchased with the idea that the asset will provide income in the future or will later be sold at a higher price for a profit.
I really don't want to turn these into an economics class but the both definition of what investments are clearly should settle the argument of whether BTC is truly an investment or not.
Truly, bitcoin became an investing instrument, pretty much like bonds or stocks, and the possibility of a good investment return is there too. And most people have been rallying the same. Although bitcoin's primal design was to be a system of payment and digital cash, time and circumstances turned it into another thing. And early adopters proved how rewarding bitcoin came to be. Thus, the concept of bitcoin playing a vital investment role apart from being a digital currency came to be.
But as many would always say, with the price volatility of bitcoin, one should invest only what they can afford to lose.