To what extent does loss of neurons in the substantia nigra affect movement?
Great questions!  Here's some quick answers, but I'll give an additional little recommendation at the end if you're interested:

How much of an effect does this destruction of neurons have on movement?


Answer:  A large effect.  The key term here is "Parkinsonism," meaning that the symptoms resemble Parkinson's disease. This often involves motor symptoms like shuffling gait, tremors, rigidity, and bradykinesia (which means slow movement).  While they typically don't result in total paralysis, these symptoms can be debilitating and require treatment, such as L-dopa.


Were these patients completely paralyzed as a result, or did they retain some ability to move voluntarily?


Answer:  Generally, these patients were still able to move, but they greatly struggled with controlled movements.  As stated above, Parkinsonism/Parkinson's disease is marked by many motor symptoms, but these do not result in complete paralysis.  Rather, they make voluntary movements more difficult and slower.  According to a paper entitled "The MPTP Story (doi: 10.3233/JPD-179006)" it appears that at least one patient was nearly (if not totally) catatonic and exhibited very little voluntary movement.


If you'd like to know why this is the case in Parkinsonism and Parkinson's disease, I'd encourage you to look up the direct and indirect pathways of the basal ganglia online.  There's likely a good lecture with helpful visuals on how these pathways work to facilitate (direct) and inhibit (indirect) movement by their effects on the thalamus and motor cortex.  Alternatively, you can let me know you're interested via a reply and I'll do my best to explain those pathways on here.  Hope this helps!