Small kernel, large surface
The right primitive is small enough to be obviously correct and large enough to be obviously useful. Phoenix, Free Router, UEA all share this shape.
Phoenix's kernel is the decision pipeline (six verbs, eleven invariants). Free Best Router's kernel is the routing loop (discover, score, route, failover, cool down). UEA's kernel is the nine stages (inspect, plan, implement, verify, classify, recover, test, generalize, hygiene).
Each kernel is small enough to be defended in a single document. Each system's surface — the lanes, the providers, the profiles — is large and grows over time. The shape is the same.
If your kernel needs a paragraph to explain, it is too big. If your surface cannot grow without changing the kernel, the abstraction is wrong.