This slander against whisper of the heart will not stand.

It's really cute and like, obviously heart on the sleeve to think that love that could last a lifetime could start in middle school. As adults does it at time come off as a little cloying? Sure.

Doesn't mean that something earnest and heartfelt like that (especially when the main plot is how to write a book or become an apprentice violin craftsman, and the struggle of artistic production/virtuosity) is instantly "unhealthy". Especially because the main audience is still kids, even if there's themes that are for audiences of all ages.

Also I'm sure you can read the story as doomed if you want as well (will she really wait for him, etc). It ends when they part ways, after all.