Simple version:
a) Bitcoins in bank = losing purchasing power daily.
b) Bitcoins not in bank = gaining purchasing power daily.
Your conclusion: The vast majority of people will knowingly opt for "a".
If the bank doesn't fail you get your bitcoins back plus interest. Without a bank there you wouldn't get anything, just saved your bitcoins...
Which increased in purchasing power. Oh noes. I only have the same number of Bitcoins but now they buy 10x as much stuff and I unlike my neighbor I didn't lose everything when the wildcat bank failed. What ever will I do with all this new found wealth.