You are going to have a ton of technical barriers in place if you are going to stop everyone from publishing to one of the many and varied code repos out there.
None of that productivity matters if the company dies due to being hacked into.
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.
