Simple: The fact that “XP” shared between between you and your crew transforms installing a fighter bay from a “no brainer” decision, to one where you need to consider its effects your short and long term goals. A fighter bay adds to your ship’s both offensive and defensive capabilities, and it comes at the expense of an optional slot. With the absurd levels of reward inflation even when SLFs were introduced, there is very little opportunity cost to installing one, especially when all other operational costs were removed or obviated by said reward inflation.
Not only does crew add a desperately needed, but still relatively minor, operational cost, but they affect how quickly you progress up Combat ranks.
Now, I’m not a fan of Frontier’s accumulative approach to ranks in general. I’d prefer a relative ranking, especially for combat. I’m also not a fan of how Frontier has handled combat difficulty. Difficulty should be relative to a system’s or planet’s security levels, government type, and faction states… not “Combat Zone” type or modified by Combat Ranking. But in an MMO, you must play the game you have, not the game you wish it would be.
With a handful of engineers requiring a specific Combat ranking for access, hiring crew represents a significant opportunity cost, especially for a non-combat oriented player. Crew represents a trade of short-term security for quicker access to those engineers. That’s what gives the decision to hire crew meaning.
Conversely, if you don’t care about that engineer access, but are concerned about how the inevitable increase in enemy ability and hit point inflation will affect your game, you can hire NPC crew early. This would be a meaningful decision as well… if reward inflation hadn’t trivialized their salary costs. But since rewards are so high, especially relative to smaller ships who can’t install a fighter bay in the first place, if you don’t care about access to those few engineers, it’s pretty much a “no brainer.”
Which is why the decision of when to hire crew is such an interesting one for me, as a player who only dabbles in combat. I want access to those engineers… or more accurately, access to additional pinned blueprints for remote engineering. Conversely, I don’t want to reach the combat ranking where NPCs become “bullet sponges.” I want to the decision to stop and fight while running missions to be a meaningful one, as opposed to a “no brainer” due to excessive TTK. So I’ve decided to hire crew once I hit Expert. Expert level NPCs currently pose a decent enough threat to my armed cargo/passenger ships, and grants access to all but one of the combat engineers IIRC.
The only effects it has on my goals is that of hampering and restricting my progress - rather bluntly, given it's a multiplier per each additional wing or crew member - which has the simple result of not making me want to engage whatsoever.
That is disengagement - the opposite of meaningful fun.
Bear in mind I'm only concerned with the impact on XP, not the credits. If crew and SLFs were made more expensive to run or somesuch, I wouldn't mind.
And the presence of a SLF, or even the act of being in a wing, most certainly does not double the rate at which you accumulate combat XP (let alone triple or quadruple it, as would be necessary for each additional crew or wing member). That's assuming you're managing to perfectly tag every target together, by the way.
"Opportunity cost" is not what makes having a crew or SLF aboard, interesting and fun in the first place. The act of having a crew, and having a ship-launchable-fighter, seeing them, using them, interacting with them - that should be what makes that interesting and fun. It bears saying that the game has done very little to make either aspect feel like much fun since the features were added.
The inclusion of this XP-reduction "opportunity cost" only directly extends the grind - if you choose to have a crew or wing. That is less fun, not more.
Trying to make up for fun deficit by introducing opportunity cost is the wrong way to go around things, unless you are deliberately aiming to entertain a masochistic audience that enjoys being punished for their participation. (As is often the case with MMORPG crowds, I've found....)
edit: Posted response and realized I had only addressed part of your response....
Improvements to combat rank gain could most certainly be made, in any number of ways. Removing XP reduction would be simpler and direct, in the short term.
It's worth noting that every MMO I've touched, I've found that as games they are ultimately not worth playing. There are any number of alternative games in the world - to say nothing of timesinks in general; there is no necessity behind playing the "game you have" if the game you have is flawed past the point of enjoyability. Settling for suck, so to speak, is not the way; it is that unfortunate mentality of commitment to a title that entraps so many gamers to an experience that I think they really aren't enjoying half as much as they would if they tried something else. And which has the compound effect of ensuring that the game they are playing, doesn't have any impetus to be improved.
I don't really see a correlation between hiring a crew member and unlocking Engineers, I think that's a stretch even at best.
And I certainly take issue with your claim that rewards are "increased". No, they aren't, they are in fact being actively reduced in every way - both your credit gain (to a marginal extent) and your XP gain (by a massive fractional multiplier extent).
There's nothing that interesting about "when" to hire a crew. It's either any time if you don't care about the loss of XP gains, or after you reach Elite combat first.
Obviously if you are only concerned with a rank as low as Expert, your concerns about XP gains are minimal.