I was thinking much the same. The two of them are in for a lot of strain on their relationship over the next decade because let's face it, you literally aren't the same person at 19 as you were at 9. Even adults change a lot during that time, and even Mimika is going to be exposed to a lot of things that'll change her own outlook. That'll be peanuts compared to what a different person Yuzumori becomes.
So to me the "I love the person you are" stuff rings hollow because in a decade neither of them will be the people they are now in significant ways. Not to mention, in Mimika's case there was an obvious physical reaction from the start, before Yuzumori even said a word, and in fact is why she tries (while comedically holding back her lust) to get to know the little girl better. You can't just dismiss that Mimika's attracted to a sexy child in those early chapters, and that look is going to change dramatically too as she grows older. Romantic ideals are one thing, but looks do play a part. And those looks will change a lot once she hits puberty in earnest.
Not saying maintaining a relationship into their mutual adulthood is impossible, but the odds are astronomically high against it.
