It is worth pointing out that, apart from Rasagiline (Azilect) the other mainstream PD drugs are off patent. So that would imply that now's the time for a drug company to bring forward something new, on patent, and more expensive if they had it.
Now, whether there are enough PWP in the developed world to make major investments in researching new drugs, that's another question. But I can't see any drug company holding back for patents to run out.
I fear the truth is more along the lines that PD is really complicated, we know a lot more about it than we did a few years ago. But still we don't know enough about the mechanism behind the neuron decay and death for a breakthrough to be certain. Sooner or later some researcher and/or drug company will strike lucky - I just hope its sooner.