If you aren't automatically rolling instances frequently, you will inevitably have some machines that were theoretically cattle but in practice are pets. (This is much easier today with k8s/mesos)

More generally, when you claim in system design that you have invariants, if you aren't using automation to enforce your invariants, they will inevitably be bypassed to your detriment by eager and/or lazy engineers. You will find this out at the worst possible time.