But if we really want to see a sustainable growth for bitcoin, we must consider it as a currency and not as an investment.
Bitcoin will never be a mainstream currency. People will always see bitcoin as an investment vehicle.
When bitcoin was invented, Satoshi Nakamoto decided to limit the number of BTC to be mined. That limited supply means that any increase in demand can make its value skyrocket. Would you spend your bitcoin if you have cash, credit card or debit card knowing about the law of supply and demand could make BTC value skyrocket few years from now? No one would ever do that, specially if you believe in the technology and you've already understand how volatile bitcoin values are.