guess you never watched Star Trek: The Motion Picture (I wouldn't blame you if you didn't) or Star Trek II: The Wrath of Khan, both times Kirk took command of the Enterprise and he was an Admiral, hell in the third one he stole the Enterprise... then blew it up, at least the Klingons did get to hold the Enterprise for a few minutes.
Star trek I, Kirk conned his way into it. For over 2 years previously he'd been star fleet's chief of operations.
Star trek II: again he was merely a passenger when Star fleet ordered him to Regulus. Infact he didn't want to take command of the ship in the first place, he just told Spock to get him there. Spock told him to take over since it was combat duty and Spock was a better teacher than combat officer.
Star Trek III: well ofcourse kirk stole the enterprise. Why he was demoted in 4.
Know what the chain of command is? It's the chain I go get to beat you with till you understand who's in ruttin command here!