the part that threw me at first is that there is no villain in the story.
once I squared that with myself, it all clicked and started making more sense and became actively enjoyable.
I think harem/poly endings are great when they make sense for the story being told and the characters therein--it's also just not always the case, I'd argue, that a poly ending is justified by the events of the narrative.
Here, though? There's no other way it makes sense, and it's not only because there's no villain/antagonistic force within the narrative. But the premise itself and the core aspects of the protagonist's interactions with the rest of the cast hinge on her being physically intimate with everyone else who's named in the series, to some degree.
Because that's a hard and fast rule, coupled with the fact that no other character is shown as getting jealous or possessive of the protagonist sharing intimacy with someone else, it would feel more jarring to suddenly introduce jealousy or possessiveness into any of the cast. It'd be throwing out their established characterization in favor of instilling drama that hasn't existed to now, and it would feel so contrived as to ruin the experience otherwise.
Up until now, it's just wholesome lesbian porn based around a woman making friends and (non-romantic) lovers out of sex workers. She's going at it all in a wholesome and non-malicious manner, as you stated, and no one is being hurt by her actions. If that were to change now, the entire tone of the series would shift, and it might as well be a different series at that point.
Actually, I will say that there is a villain in the series, and that is Asumi's ignorance of romantic love. But I say that tongue-in-cheek, and I do think it will be one of the 'big arcs' of her character that will come into play, likely after her Topping Training journey, wherein she ends up back at Mai and they have a Round 2 (or I guess Round 1 Redux).
And even at that point - I think a harem/poly ending makes the most sense. Maybe Asumi has a core partner or set of partners, but I think it's been well-established that she's not going to cease visiting the women she's met up to this point. Given they're all involved in the same general industry in some capacity anyway, there's no reason to believe most of them would cease, either. So Asumi becomes the axis upon which they all reside, and she simply spends her time with all of them and they all end up with her.
Maybe some retain an exclusively-physical relationship with Asumi, and some enter into an emotionally-active relationship as well. I think both have their merits, and showing that spectrum in this series and validating that spectrum is a good thing. But I just can't imagine any of the cast introduced thus far gets written out in favor of something "conventional", because that's one word I would say never applied to this series to begin with.
I just really love these girls 
