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?
Bitcoin can be both but it is mostly used as an investment tool for both short/long-term investment plans. Some individuals (like me) use a portion of my bitcoins and convert them to cash to purchase services/products. Unfortunately, there are only limited things that you can purchase with bitcoins and mostly the transaction fees are high. This dilemma makes bitcoin as not the preferable option to use with in paying products or services.
As time goes on, Bitcoin price will become well stabilized and then it will become a payment method only. For now, its both a volatile investment tool and payment method as it's still at its early stage and its potential continues to grow. People are aware Bitcoin will grow beyond it's current state that's why people continue to invest in it while also using it for payment but time comes when price becomes very stable and then it will be used for payment method only.
I doubt that its price would be stabilized since its decentralized nature depends its price from the law of demand and supply. From that instance, it will lose its nature and become a centralized unit, which is the antithesis of what bitcoin is all about.