Well, if I think about it, it would be weird to see the first symptoms after 70-80 % of neurons have died. Why ? This means majority of your brain would have way more neurons than necessary. Why would evolution create a brain with so many excess neurons ? That doesn't make any sense. You could say ... to have some reserve in case you lose some. But usually normal people don't lose this many neurons. And you can't say evolution would take into account old age, because we never lived this long.

Also ... I assume people have a similar degeneration of neurons during their lifetimes. At some moment PD gets triggered and this process gets speeded up. If at the time of first symptoms you would only have 20 % of neurons, it means that the person must have lost 80 % of them in a very short timespan ... It would mean that in an even shorter time period, you must lose all your remaining dopamine neurons. And this is not the case.