And no SRE would allow you to run your application in a single cluster. Borg Cells were federated but not codependent - Google's biggest outages were due to the few components that did not sufficiently isolate clusters from one another.

Clusters are probably still pets to most orgs, but the lessons about how to manage complexity still apply. Each of my terraform state files is a pet and I treat it like such... but I also use change-control to assure that even though I don't regularly recreate it from scratch, I understand all that was there.