My view on this the following: Mesos is similar to the kernel of a conventional operating system (e.g. Linux). It provides very basic services (scheduling, interrupts, device management, etc) and a syscall API. But nobody wants to program to this API. Hence you need libc or other similar libraries to provide a higher level API that programmers use to interact with the kernel. Kubernetes, Marathon, Aurora, etc are such libraries, each optimizing for a different class of applications and providing different functionality. The two (the kernel and the libraries) need each other.

I see Kubernetes as more of a programming model, not an operating system. It provides a way to express services and have them scheduled onto a datacenter. The rocket-science of how you schedule those workloads and optimize them in the same partition as other workloads is what you need a DCOS for.

In the Mesosphere world, Kubernetes is a "datacenter service" which is installed on your datacenter so that you can run Kubernetes workloads. You might also want to install DEIS to run DEIS-organized workloads. Or Spark, for Spark workloads... and so on -- all multitenant in the cluster. This is what the DCOS is uniquely good at, and why it qualifies as a true operating system.