Let's pretend that there is one piece of gold on Earth and it's the only form of currency. This currency will be needed for countless transactions. Scientists find ways to slice the single piece of gold into increasingly small pieces to buy or sell an increasing number of things. Eventually paper notes are issued on the promise of the actual gold for those who are willing to store their microns of gold in a depository or bank.
BTC is no different. Banks can turn BTC into fiat as long as there are people willing to store it there. They can offer a higher return for those willing to store it there for longer terms.
I don't understand why so few people seem to be able (or willing) to figure this out.
Let's assume that the day comes where the price of BTC stabilizes to not rise or fall more than 1% or so on average each year. Let's go further and assume that BTC doesn't rise more than 1% per year relative to fiat. If banks offer 5-6% to have it stored with them, you can bet your bottom dollar that most people will "sell out" and put it in a time deposit or some form of bank bond. If the greed and Ponzi scheme mentality that permeates this forum alone is any indication of human nature, you are on the wrong side of the idealistic argument that says that average people won't stampede toward banks with BTC just like they do with cash.