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.
Actually, bitcoin has a lot of purposes and uses. It can be used for payment transaction or it can be good for investments, but I'll really believe that it depends on us on how we able to used it. If we think that investment to it are very helpful to us then why not, we don't need to stopped it, just do what the best for ourselves.