Hi Dr Damien,
I think (or hope) that anyone who deals with PD patients knows that after a few years of dopatherapy, the patients fight at least as many side efftects of the treatment as symptoms of the disease. You mention patients not on dopatherapy, there are very few. So far, it is the only treatment and I do hope they finally find another path.
AS far as walking is concerned, drop of BP due to treatment mainly can be an obstacle. Loss of balance ( also aggravated by treatment) is another. Dyskinesia, which is definitely a consequence of dopa can prevent someone from walking. And the patient does not need to take a high dose. There are many fluctuations of state during the day, due again to the therapy and what can be possible in the morning may not be possible in the afternoon, etc. A good steady walking aid is a great help even though the patient is capable of walking just with a cane during good spells.