Must say I do miss those days when K8s was an idea that could fit in your head. The primitives were just enough back then. It was powerful developer tool for teams and we used it aggressively to accelerate our development process.
K8s has now moved beyond this and seems to me to be focussing strongly on its operational patterns. You can see these operational patterns being used together to create a fairly advanced on-prem cloud infrastructure. At times, to me, it looks like over-engineering.
Looking at the borg papers, I don't remember seeing operational primitives this advanced. The develop interface was fairly simple i.e this is my app, give me these resources, go!
I know you don't have to use this new construct but it sure does make the landscape a lot more complicated.