The API and controller loops are the point of k8s. etcd is an implementation detail and lots of clusters swap it out for something else like sqlite. I'm pretty sure that GCP and Azure are using Spanner or Cosmos instead of etcd for their managed offerings.
not exactly a fair assessment since neither of those were out and/or available to the kubernetes team at the time. sure, some things at many times from now into eternity may be or become better suited for the kubernetes data plane but at the time if etcd wasn't used there would be no kubernetes today
The Kubernetes team chose etcd specifically because they were trying to replace Borg's master/slave database at Google. Nothing about Kubernetes requires etcd; the team was trying to solve a Google-internal problem with it (and in the end, didn't gain traction within Google.) k3s uses sqlite by default which was an option at the time, other clusters today use PostgreSQL.
Have you looked at the etcd keys and values in a Kubernetes cluster? It's a _remarkably_ simple schema you could do in pretty much any database with fast prefix or path scans.
There are many forms of Kubernetes that are not using etcd that are certified by the CNCF as conformant (https://www.cncf.io/training/certification/software-conforma...). The version in the kubernetes/kubernetes repository is a reference implementation, intended to be customized by the community.
