Yes - diet and exercise can play a significant role in slowing progression. If you become aware early you can quickly adjust these things in your life and it could slow progression.
Tin foil hat: Insurance companies
Realistic: Doesn't earlier "treatment" of Parkinson's Disease yield better outcomes? I.e. dopamine supplements, etc.
Not a doctor but a relative of a parkinsons patient - the dopamine supplements are somewhat a dual edged blade given that external supplementation will further degrade your own production