In Star Trek Beyond, Captain James T. Kirk (Chris Pine) avoided the biggest mistake Kirk (William Shatner) made in the Prime timeline. When Star Trek Beyond begins, the Starship Enterprise is three years into its historic five-year mission, but Kirk is melancholy over how "episodic" his journeys have become. Kirk decides his salvation would be applying for a promotion to Vice Admiral and overseeing the massive Starbase Yorktown, a job that was nearly his until Kirk's adventure to save Yorktown from Krall (Idris Elba) changed his mind.
At the end of Star Trek Beyond, Kirk decided to remain as Captain of the newly commissioned USS Enterprise-A, and he learned the lesson that gave Admiral Kirk years of regret in the Prime Timeline. After Star Trek: The Original Series, Captain Kirk accepted a promotion to Vice Admiral and became Chief of Staff of Starfleet Operations. But this put Kirk behind a desk while he was still in his prime years, and he yearned to be back in command of a starship. In Star Trek: The Motion Picture, Kirk was jealous that Captain Will Decker (Stephen Collins) got to command the refitted Enterprise to intercept V'Ger, even though he recommended Decker for the job, and Jim maneuvered himself into the Captain's chair instead. Admiral Kirk finally got what he really wanted at the end of Star Trek IV: The Voyage Home when he was demoted back to Captain and given command of the USS Enterprise-A. Chris Pine's Kirk figured out in Star Trek Beyond what it took Admiral Kirk four movies to achieve.
Kirk Regretted Accepting A Promotion To Admiral
Aside from Spock's (Leonard Nimoy) death in Star Trek II: The Wrath of Khan, accepting a promotion to Vice Admiral was Kirk's biggest regret in the Star Trek movies. Kirk visibly loathed being an Admiral, and Spock himself told Kirk that "It was a mistake to accept promotion... commanding a starship is your first, best destiny. Anything else is a waste of material." Kirk knew his wise Vulcan friend was right, but he was blinded by his ambition to rise in Starfleet after his five-year mission ended, and he didn't realize how much he'd miss being the Captain of the Enterprise.
The first four Star Trek movies saw Kirk use underhanded means to Captain the Enterprise despite being a Starfleet Admiral. He used his rank and clout to unfairly muscle Decker out of the Captain's chair in Star Trek: The Motion Picture. Captain Spock graciously gave Kirk command of the Enterprise in The Wrath of Khan, and then Kirk stole his starship to rescue Spock from the Genesis Planet in Star Trek III. Finally, Kirk commanded a stolen Klingon Bird-of-Prey to time travel and bring two humpback whales to the 23rd century in Star Trek IV. For saving the Earth, Kirk got back his heart's desire; Captaincy of the Starship Enterprise.
Kirk Warned Picard Not To Quit Being Captain of the Enterprise
In Star Trek Generations, Kirk got to impart his hard-won wisdom on another Enterprise Captain, Jean-Luc Picard (Patrick Stewart). Once Kirk accepted that he was living in the false reality of the Nexus, he finally started listening to the bald Starfleet Officer who asked for his help. Before Kirk agreed to return to Veridian III to stop Dr. Tolian Soran (Malcolm McDowell) from destroying a star, Kirk told his fellow Captain of the Enterprise, "Don't let them promote you. Don't let them transfer you. Don't let them do anything that takes you off the bridge of that ship, because while you're there... you can make a difference." Picard heeded Kirk's advice and stayed as Captain of the Enterprise for another decade, but Chris Pine's Kirk figured all of this out much sooner in Star Trek Beyond so that he'll never repeat William Shatner's Kirk's biggest mistake.