Bitcoin can be a currency if it is stable and trusted among most of people and has a value that slightly changes with time
"A currency, in the most specific sense is money in any form when in use or circulation as a medium of exchange, especially circulating banknotes and coins." -Wikipedia
I can confirm that Bitcoin is already a currency as it is used to trade for different goods and services. For example on exchanges or like on Bitcointalk for services.
Thousands of online services accept Bitcoin as a payment method. Just due to people mindset that would prefer old school payment methods, but future is around us. Let's look into it.
The more ways to spend there are - the more adopted bitcoin is.
Thats basically it.
At this point you can spend it on SOME online resources and probably can live off of it.
We need to overcome 2 important checkpoints - when MOST of online resources will accept btc and when SOME of irl shops will accept btc