I don't see any other musician creating harmonies as rich as him. Yes, not even Miles Davis, Bill Evans or Tom Jobim (other jazz musicians that I like a lot).
It is surprising how fresh and accessible his music remains today. He never became a top hit, but also never stopped being relevant.
I can't even begin to comprehend how you can't understand that someone can write harmonically rich music.
Aside from that, Coltrane's contribution to modal jazz is significant. Though not singularly from him (or anyone else), he contributed quite a few standards and recordings to a style that was a pretty radical and rich departure from functional harmony. And if you don't consider the harmonic system this resides in (Russell's LCC is a common one used to analyze it) as "rich" then I think you might be too harsh a critic.