When you say accessible I guess you're excluding his free jazz period. And "rich" harmonies doesn't really mean anything. If you're saying that he stacks more extensions than Bill Evans, that seems hard to quantify.
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.