We seem to think Manbagi has no chance of winning Tadano's heart but she actually does. Yes, Tadano and Komi clearly love each other but neither of them have enough of a clue of that. The reason for that is exactly in the title, Komi can't communicate. One of Manbagi's best assets over Komi is that she isn't afraid to communicate. She loves to talk and Tadano does too which is why and how they've grown closer. Tadano is just as succeptable to Manbagi's charms as he is to Komi's and being able to have normal conversation is kind of a big deal when trying to establish a relationship. While Tadano is insanely dense even he has recognized cues from Komi that she might like him back. But with Manbagi she actually said, albeit accidentally. Still, he has recognized that she probably does. Another thing, even if we ignore all of this and assume Manbagi can't win his heart, Komi believes she can. What if Komi decides to never reveal her feelings to Tadano? What if she tries to hide it like Manbagi did? What if Tadano accepts her confession? The last two pages are proof that she sees that as a reality because she couldn't bear to watch what happened. She's sad because there's an honestly good chance that Tadano might like Manbagi back.
Sorry for the essay, that's just been on my mind a bit.