Alright, this argument keeps coming up, but it's honestly a really bad one. Who cares if he's not the main character anymore? What kind of story has only the main character shine, while the supporting cast are a bunch of underperforming idiots? That's not how it works, especially when you consider characters like Avdol or Kakyoin who that already doesn't apply to.
A few moments of proficiency wouldn't suddenly mean that Part III would become Joseph's show and Jotaro would be relegated to the shadows forever. It would mean that, in a couple of scenes/episode, Jospeh would have the chance to apply his Part II experience to do something worthwhile, which wouldn't be out of the norm when Part III has already devoted attention to several other characters who aren't Jotaro.