The Wormhole wrote:
In real life, the US Navy phased out Commodore in the 1980s and replaced it with Rear Admiral, Lower Half. Therefore it was decided in Star Trek that Starfleet would eliminate the rank as well.
But, but, but ... Starfleet has nothing to do with the US Navy ... nothing I tell you.
Seriously, it's very possible that the rank of Commodore still exists and we're just not seeing anyone of that rank. It's like TOS and the rank of lieutenant junior grade, there were some officers with a single broken stripe, but for the most part you didn't see them. Seems like you go straight from ensign to full lieutenant.