My counter to that is that people mostly don't roll their own, so the defaults matter. Adding more implementations just increases the total amount of complexity

My counter to that is that people mostly don't roll their own, so the defaults matter. Adding more implementations just increases the total amount of complexity going on. That probably argues for "Deployment shouldn't have been a core object type", and I think k8s folks generally agree on that now, in hindsight. But the idea of generalizing CRDs to the extreme is relatively recent.