I don't know what the situation is in the movie, but historical precedents for a newly-minted officer taking an O-6 level command are pretty much limited to battle situations where every intervening level has been wiped out. There is no organization that would gamble such valuable assets on an inexperienced and untried officer, except in dire emergency.
I pretty much agree with this- except with regards to Star Fleet not being run like any military organization on Earth today. Or else, Kirk would not have retained his command through the TOS years. He's insubordinate a LOT.
And the better parallel for Kirk is King Arthur anyway. That ship is his destiny, his Excalibur. Whether or not it could REALLY happen is irrelevant. Whether or not it makes a good story? More important, and we'll see.