Is cluster-per-app a thing people really do? Seems to defeat the purpose.

We use Mesos rather than Kubernetes, but the point is that a central infrastructure team (maybe 15 people) manages the cluster itself. It provides service owners (several thousand people) with a small and straightfoward abstraction to provision, upgrade, and decommission their particular apps.

If each service team had to deal with its own cluster scheduling, these systems would usually be massive timewasters relative to the old ways (Puppet, shell scripts, etc).