I'm not sure anyone knows the answer to your question at this time. However, it may not be so complicated. While the new stem cells may help in the regeneration of new dopamine producing brain cells, they would be doing nothing in terms of stopping the cause of cell death in the first place.

Take the following hypothesis as an example:
1- Some combination of genetic susceptibility along with environmental toxins causes a protein(s) to misfold.
2 - The misfiled proteins uptake to a certain part of the brain, like the olfactory bulb. The misfolded proteins take on a prion-like nature and travel from neuron to neuron, changing the structure of the proteins in these brain cells.
3- Mitochondria in the infected neurons are not capable of removing the misfolded proteins from the cell, causing a protein aggregation which eventually causes cell death.
4- Stem cells are inserted into an area of the brain which has had a significant amount of cell deterioration which has led to serious motor and non-motor symptoms. The stem cells stimulate some cell regeneration over time, which leads to a decrease in some or all of the symptoms.
5- Nothing has been done up to this point to actually remove, block or prevent the continuous prion-like uploading of the misfolded proteins from adjacent infected cells.
6- Ultimately, the new, dopamine producing cells, will get infected with the misfolded proteins, which will aggregate and eventually die, just like the original cells.

Obviously, this is just conjecture. However, there is a growing base of animal research to support the premise. There have been mouse studies on neuronal grafting. The mice had PD induced. Then, they had clean cells grafted into their substantia nigra, which resulted in significant reductions in symptoms, which eventually returned. Autopsies eventually showed the same Lewy Body buildup as the ungrafted cells.

I personally think that stem cell and growth factor cures, if ever found, will only be effective for a limited period of time. Symptoms will return if the actual cause of neuronal deterioration is not stopped. Just a thought......