Except that's not entirely true... At least, that interpretation of the story would've been a lot easier to defend if it wasn't for the special chapter 3 of volume 9 in particular. But in that chapter Sakurako explicitly brings up the idea of them dating, and then immediately regrets speaking her mind to Kasumi. On top of that, we see Sakurako be possessive and get jealous of other people that approach Kasumi, and she shows sexual attraction as well (in particular there's a chapter where she gets drunk and asks Kasumi to sit on her face). This all paints a picture of Sakurako wanting more out of their relationship but avoiding actually talking about it out of consideration for Kasumi and her detachedness, just as Sakurako always has been shown to go out of her way to serve and be considerate of Kasumi to extreme extents.
Honestly, when I tried to remember how the series ended, I seemed to remember that Sakurako's question at the end of volume 9 gets almost completely forgotten and dismissed by Kasumi. That sounded absolutely crazy to me in my head, so I had to go back and re-read the ending to confirm. And while technically that kind of is what happens textually (Kasumi has one line where she tells Sakurako she's "seriously thinking" about it, and then it never gets brought up again in text), I kind of think the subtext is that they started officially dating offscreen? I think that's what chapter 74 is basically meant to imply... That being said, I do think the message is supposed to be something along the lines of, "the label doesn't matter, what matters is their relationship and the life they create together," but personally I kind of find the message to fall a bit flat, given the aspects of Sakurako mentioned above. In fact, chapter 74 is kind of a great example of why defining their boundaries and expectations is important for their relationship: Kasumi starts getting hit on and Sakurako only happens to find out about it and intervenes due to happenstance, after becoming jealous.
Now, to be honest, I just saw Futaribeya mentioned and wanted an excuse to talk about it

, but to bring it back to this manga so my comment isn't too far off topic, I honestly kind of find Sakurako and Kasumi's dynamic
more toxic than Fuyuki and Eri's up to this point? As crazy at that sounds... It is an apt comparison, as they both have a very similar issue where Fuyuki/Sakurako clearly have feelings for Eri/Kasumi, but avoid bringing it up in order to maintain the intimate relationship they've already achieved with them. But I think the key difference for me is that Eri is explicitly lead to believe that their relationship is the way it is by Fuyuki, and Fuyuki doesn't push past the boundaries of that established relationship, even if she wishes she could. Eri and Fuyuki have had a defined relationship with clear expectations and boundaries, and once it has been communicated that Fuyuki wants different expectations and boundaries from their relationship, Eri seriously considers them and communicates openly with Fuyuki about how their relationship can progress from that point. In contrast, Sakurako and Kasumi have a relationship with no clear boundaries or expectations, and Sakurako regularly hints at wanting more but Kasumi never seriously considers those hints to have any deeper meaning (until arguably the very end of the story). Not to paint Kasumi as the "bad guy" of her story either. I think that similar to my argument for Fuyuki and Eri's predicament, both Sakurako and Kasumi have partial responsibility for this disconnect and it's largely a fault of communication.
That being said, there is the argument that Sakurako and Kasumi's relationship is consistently portrayed as positive and fulfilling for both of the characters. I'd argue that there are numerous examples where these lack of definitions cause Sakurako distress, but one could also argue that it's mostly just played off for laughs and Sakurako is content just being with Kasumi in an undefined "situationship". While that may be how the tone of the manga portrays it, I just have a hard time fully buying into it. While I feel confident that things would work out for Eri and Fuyuki in the end, I don't really get that same feeling of confidence from imagining Sakurako and Kasumi's hypothetical continued relationship, at least in what we're shown of it. Rereading those last few chapters, it does feel like they probably do have moments of open communication between chapters, but we're never shown that or how they actually deal with their expectations the same way we're being explicitly shown Eri and Fuyuki doing that in this chapter. You could argue that I'm silly to doubt the strength of their relationship when the manga only ever portrays is as strong and lasting, and that's fair, but it doesn't really change how it makes me feel reading it. Yukiko certainly tries to portray their unspoken arrangement as this profound relationship that could only be lessened by labels, but it doesn't convince me for whatever reason, and the framing itself seems to become inconsistent in a few key parts of the manga in my view.
Anyways, I'm sure that my takes are going to be pretty controversial if anyone bothers to read through it, but seeing Futaribeya mentioned unleashed a rant inside of me that I didn't even realize I had stored up. My bad