Daystrom Institute

Likewise mostly Enlisted people would be spending a few years at most and moving onto other careers in civilian life as most people don’t want to be in the military forever and if they do they become officers.

We see this bear out in Miles O’brien. He enlisted in starfleet rather than attending the academy. He climbed the ranks (mostly in backstory) and by the time we meet him in TNG he’s a chief petty officer, non-commissioned but officer nonetheless. When he accepts the promotion/transfer to DS9 he achieves the rank of senior chief petty officer.

Its hard to say for sure that the rest of the show is inaccurate (or otherwise) in this regard because the shows mainly focus on bridge officers. Many characters receive offscreen promotions throughout the shows and you have to pay close attention to their uniforms and rank insignia to catch it. But also, you have to bear in mind how narrow a view we get of any crew outside of like Voyager.

For example, the Enterprise-D, as a Galaxy-class starship, can have a crew complement of anywhere from 1,000 to 6,000, and TNG as a show overwhelmingly focuses on just 7 of them. “best” case that’s a view of about a half a percent of them, so it would be easy to draw nonsensical conclusions if you overly extrapolate from that small percentage. Maybe a lot of crewmen do actually retire after just a few years, it’s hard to say for sure since there is basically no evidence either way.

Basically everyone in the commissioned officer corps (ensign and above) attended the academy first in order to receive the officer commission. You would not go to officer academy in real life if you did not intend to dedicate your main career to being a military officer, and likewise, people who attend starfleet academy generally intend to dedicate their careers to starfleet. Every depiction of the Enterprise is that it is the federation flagship and so we can consider a station there as being highly sought after and likewise regarded, it represents a high potential for being a crowning point on anyone’s career, so it kinda makes sense that the core officers might be reluctant to trade in for an inferior assignment, even if it meant a bigger promotion. Another aspect of this is loyalty: most of the officers depicted across all of the shows are supposed to be stand-out talents among the federation, with solid leadership skills that clearly foster loyalty. Loyalty that can create a reluctance for too much change. We see this reflected in Riker’s long resistance to getting promoted off the Enterprise to Captain. He doesn’t want to, partly out of loyalty to Picard, but also partly out of loyalty to the officers that report to him, and by extension the rest of the crew, and partly out of the chance that his potential new command is simply not as cool as being the first officer of the Enterprise.

Voyager has different circumstances, of course: the crew complement is much smaller, and they are stranded, but Rick Berman also decided that field promotions would be unlikely in their circumstances since they have no real way to gain more crew

There would be fewer officers

Important to note that even among the enlisted ranks, the NCO corps begins just one rank above crewman. Similar to real life military there are only 3 ranks of non-officers