I do want to defend the ending as being the closure of one moment of their lives and opening of another. They are only starting HS so still many unresolved problems (careers, life together, etc), which is understandable.
That being said, the series hasn't really moved me like earlier on. The trip to the theme park, the speech itself seemed much more muted than Ichi giving his speech or the condom episode.
Maybe she wanted to make it clear the series is over by leaving us with a lower note? I dunno, but something intangible changed that made the series less rewarding. Hard to put my finger on, but I almost feel this particular ending could have been made 20 chapters ago.