At some point in time I guess it will become either one thing or the other. It cannot stay both at the same time. Can it? Is it a healthy usage method to have it as both?
But this is exactly what has been happening over the past 10 years?
What the question really is I think is how much volatility is hindering adoption. Bitcoin as a payment method will work regardless of its value, since the network is completely distributed. Even if prices drop to close to 0 one day, as long as the network which bitcoin runs on stays the same, there won't be any issues with bitcoin being a form of payment. For example, merchants accepting bitcoin in a period of volatility can still do so, through hedging their exchange rate risks with a third party or by themselves.
I do think that speculation will lead to somewhat decreased actual consumer use, but at the end of the day, will not completely stop consumers adopting it. And as I expect bitcoin volatility to continue to drop in the future like it has, it's not a particularly big problem.