The fact that you don't allow anything to connect to etcd besides the K8S components doesn't really make etcd itself inherently secure. It also implies that access to etcd is an all-or-nothing proposition, which doesn't work well in complex environments. People will need to get into etcd in order to perform debugging, maintenance, etc., and at big organizations, they will need differing levels of access depending on their respective roles.

And your assumption that only K8S will access etcd isn't necessarily shared by others. In the model implementation, perhaps that is so; but in the real world, implementors may desire to share etcd with other applications. This too necessitates additional security options for etcd itself.