The epilogue is actually anything but open when one takes it in the context of the series. Remember her condition: ‘when you get older, and your feelings still haven’t changed, let me hear you say that again.’ No matter how his friends goad him to pursue Sakurai (very possible to be even more attractive), he stays mum on the matter. He also didn’t arrive with any girl. Moving is something major, and I think if he really had a girlfriend she would be around to help him do it. From their conversation, however, it doesn’t seem he’s seeing any girl (again, he’s being goaded by his friends to see Sakurai).
Kouta quips in the ending that ‘as long as I don’t forget these feelings …’ and Natsuki reaffirms this with ‘Definitely some other time.’ In the epilogue, upon his return to town, Natsuki lay in wait at exactly the same spot with a very smug, knowing smile. This is in contrast to the time where they first met, where she talked to him with a straight and serious face. The ‘I love you’ in white background doesn’t even have to specify whether it was Kouta or Natsuki who said it, just that it was said and that no matter who said it would probably still mean the same thing (it’s probable even both of them said it!).
Although I wish just like any other guy for a better closure, this was done very well for a discontinued manga.
(Yes, I actually like Mizuki Kawashita’s works. I hope this gets a proper anime treatment by Kasai Ken’ichi and J.C. Staff.)