Honestly, if we're being realistic, almost none of these girls end up together, that's just not how high school relationships work out.
I absolutely agree with your whole post, but personally I feel that, while it wasn’t necessary to show that all the pairings worked out long term, it would have been wonderful to give them all present-tense closure - a confession, a kiss, a sex scene, as befits wherever they are in their relationship and what Kishi-sensei felt like drawing.
Maybe they aren’t going to (or shouldn’t) grow old together, but at least they (and we) would always have that moment/memory of their school romance blooming.
For those of us who have spent 15 years caring about these characters it just… would have been nice.