I do wonder if Kirk actually intended for Sonak to eventually become first-officer once the mission was over. If Kirk was planning to stay aboard, it's likely he'd have needed another first-officer if Decker was leaving.
Note that, in the Real World, Sonak was only created so they didn't end up wasting David Gautreaux's research into creating Xon. (Although they did
end up wasting him.) Sonak was always slated to be indispensable.