When we first see Dr. M’Benga (chronologically), he is the Enterprise’s chief medical officer. When we see him again, he is still on the Enterprise, but working under Dr. McCoy’s command and, if he was on the ship at the time, under Dr. Piper’s command as well (Dr. Piper was CMO in Where No Man Has Gone Before).
So why do you think he was demoted to a secondary position? Was it something to do with his actions as the Butcher of J’Gal or his killing of Ruh? You would think he wouldn’t be on the Enterprise at all if so. Did Pike demote him? Did Kirk demote him? Was it on orders from Starfleet Command?
What do you think was the reason why McCoy became his superior officer?
Well whatever it is, Pike and the radioactive accident had something specifically to do with M’Benga stepping down. I wouldn’t call it demotion necessarily as there are reasons that someone could willingly step away from the position of Chief and still hold their rank. Either way, during the Quality of Mercy episode M’Benga was still CMO under Captain Pike.
With how M’Benga holds himself and takes the quality of his patients personally, and becomes heavily emotionally involved in his work, I think he stepped down willingly because he no longer believed himself to be capable of providing the support necessary to a Captain. I think maybe he worked on Pike and tried to save him but when Pike was too far gone that M’Benga became a bit despondent. We know the two are friendly and decently close so maybe between being unable to save his daughter and being unable to save his Captain he just felt he wasn’t capable of doing his best anymore. Still wanted to help and assist and thought of the Enterprise as home, and wanting to stay near his family in the crew, but didn’t want to carry the weight of that responsibility anymore.
It certainly could have been a voluntary “demotion” and there is a reason to believe he hasn’t changed a huge amount. There is an issue that needs to be resolved though:
On the one hand, he still seems to have a streak of violence in him the way he slapped Spock (at Spock’s request) in A Private Little War, which would track. On the other, you would think he would have worked with Spock in the plan to get Pike to Talos IV.
If we are to believe the two M’Bengas are the same person, which we have no reason to disbelieve (we can excuse the TOS M’Benga’s lack of an accent), an explanation for why he wasn’t part of Spock’s plan would be an interesting thing to ponder.
This seems likely given that in the timeline where Pike was still in command in 2266 having avoided good accident M’Benga was still CMO. I would imagine that having give through all that shit he wanted to step back a bit and let someone else run the department.