It's unclear why we need two server commands for this (except now for backward compatibility; I'm not proposing breaking that).

Offhand, unless there's important that Initiate does that Reconfig doesn't do, as single might as well do whatever Initiate does if the existing nodes don't have a replica set configuration.

(I admit this is just syntax, but it's just another detail you have to keep in mind, have to teach people, and can get wrong.)