We have a small experiment with that already - corporate NATs which are supposed to achieve the same thing. They still get owned. Even SWIFT which is basically as isolated as it gets in business had bad actors with access.

I don't believe a complete separation would ever be possible. We'd have to put banks on the commerce side. But that means every single business entity would also have to have access to them. But most businesses also need access to the other side and will not do perfect separation.

In malware research labs you can find rooms literally painted in two colours to separate the isolated part and prevent joining networks by accident. Normal companies do not care that much. Most single-person entities will just plug both ends into one computer and go on with their life.