One could inject a scheduler in between, by emulating a single machine's remote API, but that would still lack an explicit model of what the user was trying to instantiate, the equivalent of the Pod template in Kubernetes.
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The decoupling of Pod, workload controllers, and Endpoints, and a precedent for multiple workload controllers in Kubernetes has proven very flexible, for supporting many, many types of workloads. There are now many application-specific workload controllers (aka Operators).
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Explicitly representing the PodTemplate as a separate object, as proposed in http://issues.k8s.io/170 , may also have been useful for these third-party controllers, but in practice the lack of support for that hasn't been a huge obstacle. (Well, the API exists, but is unused.)
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I proposed the idea of modeling workload controllers as loosely coupled sets of instances grouped using a label selector in June 2013, based on an 11-page analysis of Borg Job use cases, around the same timeframe as the original labels proposal.
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That partly inspired http://replicapool.googleapis.com also, though the lack of labels in GCE at the time made implementing the full model infeasible.
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Aside on "template": a "template" is a pattern used to make copies of the same shape. I think the Kubernetes "Pod template" usage is true to that colloquial definition, but typical CS usage implies parameterization and/or macro expansion, so maybe "prototype" would be better
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The idea of explicitly modeling state so that it can be externally controlled and observed is a key principle of Cloud Native. I originally included it in a longer form of the definition I wrote for CNCF: https://github.com/cncf/toc/blob/master/DEFINITION.md …
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The principle can also be applied to workflow systems and configuration management (e.g., see https://github.com/kubernetes/community/blob/master/contributors/design-proposals/architecture/declarative-application-management.md …). Embodying these as code is powerful, but with great power comes great responsibility, since it obstructs external tooling and automation
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