This is beautiful, in that aching, nostalgic way.
I understand Kotori's motivation here, though it's more than what she's saying, particularly when she's watching Umi compromise her own goals to match Honoka's potential. The scene with Nico is interesting — it feels like something of a reference to Manase's "Second Years", elsewhere on the site, where she's unsettled by the trio's closeness and Honoka's possessiveness.
Given that Umi and Honoka are each, ultimately, inheriting their family businesses — blocks from each other — I rather suspect they, at least, won't stay separated; but then, the two of them together, without Kotori, could end up feeling like — like a missing tooth, if you catch my meaning? You can't help but feel the absence, probe at it, whenever you become aware.
