I heard a talk about this at a scientific conference a couple of years ago. The speaker has both a PhD and a medical qualification (impressive!) and he made the point that PD is probably not "one disease". Rather, it is a multitude of conditions with some clinical signs in common.

He drew an analogy with cancer, which is also not one disease. Because of that there is not a single "magic bullet" cure for cancer and there probably won't be one for PD either.