The scenario for a lack of sufficient Alzheimer's research is currently what we're living with: 83,000 deaths a year, and approximately 5 million patients living with the disease, in the US alone. Probability: 1.0. I don't know if it's "worst-case," depends on whether the disease ratios are progressing year-over-year, and I don't have those numbers at my fingertips right now.
I have yet to hear a realistic strong AI nightmare scenario (i.e. one that doesn't presume a magically-smarter-than-all-of-us-combined AI as a McGuffin with no solid functional grounding) with a probability anywhere near offsetting that cost. Besides, stacking the constant chronic cost of a few tens of thousands of deaths against a nonzero-probability species-ending scenario is nearly an apples-to-oranges comparison---by the logic of simple probability-to-risk modeling, we should be stopping all disease research right now and pouring 100% of that money into fast asteroid detection and mitigation solutions.
Practically speaking, I think we have plenty of time while we are doing the research to cure a current and real disease to puzzle through the detection and mitigation strategies for a strong AI threat that is---at best---decades out (and, some would argue, will come to pass inevitably with or without our Alzheimer's research, yes?).