SDN's are an artifact of classic networking mindsets. There is no air gap, pretending one exists by creating a virtual one is just being blind to the realities of network gear.
We need to move beyond SDN and embrace a point-to-point world where trust is established through pki or similar means. SDNs give a false sense of security, add a ridiculous amount of complexity to routing, and create confusion when it's unclear if there is a hardware problem or a software problem.
SDNs are only there because people aren't innovating enough in other layers.
