I see bitcoin as a tool or instrument.
The fact that the value change (positevely) over the time is also the sign that this is a strong medium and not only a currency.
I can understand why people see bitcoin as just an investment. You can consider everything just "an investment".
For sure you're not buying your house as investment but as tool for living
Of course it can change in value (becoming more valuable) but in the mean time you are using for living and many other things.