Just goes to show that Starfleet is ruthless towards faulty individuals at lower positions already
Definitely a thread that carried through to TNG. There is apparently a lot of ambition and jealously in Trek. Spock was always being accused of attempting to take over the ship. Mitchell was obviously harboring some jealously and resentment toward Kirk. Picard has the "nightmare" in threads of having to find out what his life would've been like if he had not been promoted. He was dressed down by Riker for being unfit because he'd never taken any risks (You're a chicken!). Riker gets his turn when a new XO shows up to bump him out of the way.
For a society that is supposed be "above it all," they really play cut-throat at the promotion/hierarchy game in Starfleet.