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.
Unless the implementation has bugs. Then, the sensitive area is just a single zeroday away. Airgaps don't have that attack surface.
I agree with your previous point that we should encrypt point to point, but that's one layer up from the problems SDN is solving.
Just for clarification: That was written by bluejekyll, not by me.
They add extra failure modes and reduce throughput by decreasing MTU. This is almost all because people want virtual networks with their VMs, rather than just embracing IPv6.