Speculation only, but...
In modern militaries, there aren't that many opportunities for promotion at the higher levels, as a promotion would be associated with an assignment known to be in need of an officer, and a modern Navy doesn't have anywhere near as many ships as Starfleet, nor as many sector command "starbases" or comparable positions for flag officers. On the other hand, military men on the lower rungs of the ladder are running out of viable working years as their physical fitness quickly deteriorates. Promotions would be carefully preplanned by the organization to meet its needs, and anybody arguing against them would justly raise the ire of the organization. In most cases, it would be tantamount to disobeying an order; a promotion essentially is
a transfer order of sorts.
Starfleet would be facing somewhat different conditions: a huge number of ships, suffering constant losses even in peacetime, would create job openings, whereas the people queuing up for the openings would remain physically fit for longer thanks to future medicine and whatnot. Extensive cross-training would allow redshirts to take promotions to goldshirt positions etc. And the long-lived people might spend decades in a given position, accumulating merit within that position rather than by climbing up the hierarchy. Plus, the pressures from below would be reduced as there apparently would be no pay incentive to earn a promotion: you wouldn't need to get extra brass on your collar in order to feed your family, and your motivation to hunt for a promotion would probably come from you aiming at a specific ideal position. Once you reached that, you wouldn't be bumping into the people right above you in hopes of taking their jobs and pay rates.