I assume that in the beginning the endgoal was selling the slaves, but as the community is constantly clamoring that they want to keep them all - that might change. Nut in my opinion it was never meant as a primary slave management game, but an RPG , where it was one of the big mechanic. I was very surprised in the beginning how clingy people are in a game like this.
Maybe because without "emotional investment" there is a lot of other, non-porn games, with a much better budget, that would scratch that itch? I mean, hell, if you take away the sex parts that's pretty much how you can play a game like, say, Crusader Kings 2.
Besides, the more possibilities a game provides to drive a character into specific direction, the more "mine" they become.
The only problem for me is, as you said, it devolves into a soulless economic simulation after a while, with no end goal.
For me, as soon as I start hitting 30+ slaves, I know I'm about to put the game away. Even with all the new custom rulesets you can set up with the mod-that-shall-not-be-named.
Even if you play on highest world deterioration/economy settings, the game is not difficult. It's mostly deterministic, which basically just means you're competing with yourself how fast you can accomplish certain goals.
Plus the text based engine becomes a chore to read... which is a problem with all text-based games - not the most visually attractive and engaging when played. You can look at a picture of a naked ass all day and never get bored, but you will get bored reading 'naked ass' every second paragraph.
The mod-that-shall-not-be-named somewhat recently got a Daz3D models included. It's still pretty basic, but does complement the rest quite well.
On a side note, it is interesting that lewd games have only adopted (or perhaps 'pornified') a limited number of gaming 'engines', mainly RPG with its 'improve your slut with (bad) experience and outfits/gadgets' logic or more or less branching VN. I wonder why that is.
I'd definitely chalk it up for desire for some "emotional investment." You either play for "I make them into 'personalities' I want, and they are mine" or the story - which by itself requires some emotional hooks.
We are sad, lonely, and pathetic creatures. Now give us more of the kind of games we like >.>