> Why would we not want to be able to discern life-threatening information sooner?
Because there's a good chance you'll receive a treatment that will cause you to die sooner than had you not known about the disease for another decade. C.f. why they pushed back the recommended age for mammograms this week.
1) Wouldn't you want to know say a year in advance, before you made all kinds of hospital visits and it was finally confirmed? I bet you would. Perhaps to start preparing for a different life, career, perhaps to advance future plans of things you'll gradually become less able to do. People want to know.
2) Can you imagine that if somehow, for example, you could detect Parkinson's by smell, that this would open up all kinds of findings, research and understanding about what Parkinson's is, how it works, how it's detected etc, that could potentially lead to better treatment or even a cure? I bet you can imagine there's a positive correlation between understanding something better and the ability to treat it in future.
3) it's simply interesting in and of itself. How curious, isn't it?
Looks like you never knew anyone that had a terminal illness.
And regardless while there isn't a cure per-say for Parkinson's there are treatments which can delay it's progression and the earlier you get them the more time you have.
Not to mention the value in correlating another physiological change with the disease. Maybe research into how this works can get us closer to a cure.