a very good point and i completely agree with it.
but it doesn't have to be only one. you can do both. you can use bitcoin as a currency but also have it as an investment. for example you buy bitcoin today at $1 to be able to spend it as a currency when it is worth $2 which satisfies both investment and currency aspect of it.
the price rise, the volatility that comes with it and the fact that people will use it as an investment is inevitable.
I agree with you, but let's look at it this way, just now the value of bitcoin is determined by the number of demands and supply. Meaning if the supply is more than the demand, the value price drops. Bitcoin as a currency will not be affected by this particular factor. As a currency, a fix price value will be used if you using fiat to exchange for bitcoin. The decentralization idea behind the creation of bitcoin was to make the transaction of funds unlimited and not controlled by an individual or groups or authorities.