Even in a complete compromise of banks systems you are not going to be able to run off with all their money. You need to get your transactions into the batches

Even in a complete compromise of banks systems you are not going to be able to run off with all their money. You need to get your transactions into the batches somehow. And the batch needs to still pass all automated checks. Then you need to stay under the limits set for the bank on the correspondent account at the other bank / clearing house. Banks know this and really don't have nearly good enough security practices for running a bitcoin exchange..