This "chicken and egg" question occurred to me while reading an interesting article [1] concerning Amyotrophic Lateral Sclerosis (ALS).
I also saw parallels with some aspects of PD research.
"These proteins end up collapsing the cellular clearance systems, which finally leads to the death of the motor neurons."
"We must find ways to reduce the production of ribosomes so that waste decreases, while still keeping a sufficient number to guarantee the correct functioning of the cells." (c.f. the posiphen approach to reducing alpha synuclein)
[1] MedicalXpress article: Accumulation of 'junk proteins' identified as one cause of aging and possible source of ALS.
Trials coming from Zhittya 
Diluting blood plasma rejuvenates tissue, reverses aging in mice
Clinical trial suggests Parkinson's drug is safe in humans
Why exercise slows progression of PD: reduces α-synuclein aggregation
Safety and Efficacy of TUDCA as add-on Treatment in Patients Affected by ALS Trial
