It doesn't help that enemy fleets are always allowed to have more officers than you. I have 10, but I can't tell you the number of times I've run into hostile fleets that had 16, meaning capturing objectives in combat is essential so you can try and match their deployment limit.
Sure I can hire mercenary officers, but that then means dedicating my story points towards hiring and keeping them, and making sure I'm always doing things to optimize my experience gains to gain more.
Yeah that's definitely an issue. But that at least is made up for by the player having access to s-mods and much much better (generally) ship loadouts. It's still a big hurdle to overcome.
Another aspect of this issue is when attempting to switch fleet comps. Is it really desirable to have to start a totally new playthrough to make ship changes? Maybe a broadly sweeping change, perhaps. Even just moving to 3 or 4 different officers is a big SP investment between mentoring and making elite skills. And they might not even get the skills I want (a topic for another thread). And I have to delete officers I made that SP investment in to do it? What if I don't like the change and want to go back?
I feel like with the ability to keep reserve officers, adjusting the fleet comp becomes a lot more friendly to do.
There are certainly Mercs available to compensate for this, and that's a relatively decent way to approach it, but at scale, that's a massive amount of SP investment every cycle. I'm not sure how sustainable that is. When I was playtesting stuff before the .97 release, I just gave myself a billion SP to experiment so I didn't really get a feel for how it would feel to have half a dozen mercs wanting SP every cycle.