Making Kirk a "conventional" officer would seem to be counterproductive, as it throws 1960s verisimilitude right down the drain - barely-thirty-somethings don't command the cream of the cream of the cream in any real navy if they ascend a "conventional" career ladder.
Except there's no indication that the TOS Enterprise is in fact the "
Crème de la Crème." The Enterprise could be just another medium sized cruiser. In a fleet where such crusiers (in various classes) existed by the hundreds.
We do know that the Enterprise is a older ship, it's the only example of a Trek hero ship that wasn't a new construction.
And she does seem to get a fair number of little shit assignments, along with the occasional glamour job.
Based on uniform insignia, Kirk
might have still been the rank of commander when he first took command of the ship (WNMHGB).
Stephen Decatur was 25 years when he made captain, and Star Trek does have a strong connection to the age of sail. Starfleet is also shown to be a dangerous profession.
Constantly creating room at the top?
Modern day US Navy would be like this ...
4 years after commissioning to make Lieutenant
5 plus years to make Lieutenant Commander
4 plus years to Commander.
So while Kirk's advancement was rapid, it wasn't freakishly fast.