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.
Yes, as of this time when bitcoin is not yet used for all the transactions as what the fiat does, bitcoin is still better for investment. But as the time when it will be used for all transactions, then it will be as usable as the fiat.