One of the problems I've heard with Mesos is that it would require your application code to need to use its constructs - this may not be true or inaccurate now.

One of the problems I've heard with Mesos is that it would require your application code to need to use its constructs - this may not be true or inaccurate now. For a K8s pod, you create a container and it doesn't need to be rewritten unless it's stateful and is not really a 12 factor design application. Mesos' originating use cases come from stateful big data ish systems like Hadoop and Zookeeper clusters, and K8s aimed more at containerized web application infrastructures and both are filling out use cases the other direction nowadays. From a risk perspective, unless something terrible happens with K8s adoption soon in 4 years I expect every other shop to be using K8s in production and many, many more people will know how to run it day to day than they do now. With that said, adoption is quite slow in some places and nothing is assured.