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.
I also believe that bitcoin is an investment, a really great investment because our money we will invest with bitcoin can grow reallly high in the future, that's why if we our the investor we really need to trust the development of bitcoin every year.