Surely in a bank, when publishing, the decisions aren't down to a single dev. Either many people made a mistake together or many people made lots of small mistakes separately that added up to one big one??
FWIW, I don't know, I'm a small time potter, just how I imagine things happen in banks (I do know a couple of devs who work/worked for banks).
You might not even know that this happened. Mysteriously, a clone company pops up, or a competitor seems to know all your contract negotiations and customers.
Another possibility is that a lawsuit over data leakage takes out the company. Sometimes reputation alone can kill a company.
BTW, it's really pessimistic to suggest that an air gap would kill productivity. It isn't that hard to physically transfer files from the computer at your left to the computer at your right. The disconnected network can have copies of popular software repositories, including for your OS to get updates.