I'm no expert and am open to be corrected. I believe Levadopa can lose its potency after 5 years. I'm on 8mg Ropinerol (dopamine agonist). I feel it just makes me feel 'normal' at the moment. I'm trying to not take levadopa for a while longer although some symptoms like stiff hand and foot are worsening.
The thing is everyone is different with this disease /disorder /condition. That makes it so hard as one size does not fit all.
Diet and exercise and positive outlook definitely slow the progress I believe.