Say BTC does become the currency that people rely on. How would a loan system work? I would imagine people would still need to take out loans for large purchases, such as cars and houses.
Without a credit system how does something like that play out? Would credit cards in BTC exist, and you still received a credit score that allowed you to take out money for a house? The fluctuation of the BTC price definitely makes it difficult for that model to work.
If Bitcoin becomes the currency that people depend on, then a credit industry will appear. It is probably safe to assume that Bitcoin cannot become mainstream with one.
What bitcoin brings to the table is a algorithmic check and balance which prevents fractional reserve banking. You can't lend $10 for every $1 deposited. You could lend 5 BTC for every 5 BTC saved and the incentive to do so would be an interest rate commensurate with the risk of the borrower defaulting. If our banking system had run on a one-to-one ratio of depositors to borrowers, and proper credit checks, there wouldn't have been a problem.
There is no reason why fractional reserve banking could not be done with bitcoins. After all, it existed when the U.S. was on the Gold Standard.