All of these companies have been operating as if keeping heaping wads of cash behind the counter was fine, merely because it was convenient.
Then implement the vault as offline storage (encrypted files burned onto DVDs and physically stored in a vault for example).
You wave your hands in the air and say 'they wait', but there goes all your speed and cost advantages.
A cash register has things incoming and outgoing. It's not constantly draining all day. You might need someone to run to the bank to get a bunch of pennies or a bunch of singles to make change.
(And with the "cold storage" concept, it's free and instantaneous to move money into your own cold storage whenever your till gets over a certain amount.)
edit: some small percentage of customers would be a smarter measure than some specific number.
You're still making a fundamentally invalid comparison: with cash, your security threats are still limited to people who are nearby and have both the time and means to move large amounts of currency. Bitcoin allows anyone in the world to steal amounts which would require a large team with dump-trucks in the real world even if the bank completely screwed up their security design.
http://news.yahoo.com/mtgox-opens-call-centre-500m-bitcoin-l...
That's a lot of duffle bags to haul.
What we're actually talking about, however, is like being able to teleport into a bank anywhere in the world, wave a magic wand which converts everything on the premises into tightly packaged $100 bills, and teleporting back out of the country. In the real world, running out the door with a bunch of duffel bags and people shouting tends to attract a lot of attention and make escape a lot harder than closing a network connection.
That's because there's a huge security net provided by the general public against the fuck-ups of banks, no matter how big.
Oh, I don't disagree with him/her.