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 think BTC is like a currency in general, where someone can make bitcoin an investment or not. In my opinion, it is only truly immovable physical property that has investment properties such as land, houses, etc. So in my opinion, bitcoin depends on the wearer just wants to use it as an investment or not.