> Then before you know it, the devops folks have decided that they need to put a gazillion other services and an entire software-defined networking layer on top of it.

I'm not familiar with kubernetes, but doesn't it already do SDN out of the box?

> doesn't it already do SDN out of the box

Yes and no. Kubernetes defines specification about network behavior (in form of CNI), but it contains no actual implementation. You have to install the network plugin basically as the first setup step.