Doctors who go to starfleet academy (when it's well established McCoy never did - just like the service today)
I don't have time to readddd through this whole thread, but just refresh my memory as to when this was established on screen in the original series or films. Not books, interviews, unfilmd scenes or conjecture based on things open for interpretation.
Cuz I hear this a lot, but have never seen this said.
The Ultimate Computer - Spock has to explain a term to McCoy that is used by midshipmen at Starfleet Academy
The Menagerie - Spock surrenders himself to McCoy as ranking officer, but turns command over to a line officer (I forget who now).
McCoy, for his part, has no idea what to do with Spock.
There was also an often cut line somewhere else, which I only saw once, which was even more explicit, but I just don't remember it, and I don't ask anyone to take my word for it.
Aside from all that, the extrinsic evidence (Rodenberry's modeling the functioning of Starfleet after the US Navy of WWII; some language (I forget just what) in the writer's bible) is incontrovertible.