I think years ago they did transplants in PWP in Canada, and it was as you said, symptom reduction for several years, but ultimately a return to full-blown PD. Autopsies revealed the new cells had succumbed to the same fate So those therapies will either need to be repeated when their therapeutic benefits begin to wane or....the cause found.

I know of one PWP who had a second DBS suggested to him when the benefits of the first one were no longer evident. I had no idea this was an option.