If every human on the planet died the planet would largely continue on, if every insect died then the planet would go through a large catastrophic change. "Animals" aren't irrelevant, and the human brain isn't everything.
Insects and plants are closely tied together, with insects being essential for the flowering of certain plants.
Are mammals so closely tied to any species outside of the class?
It instead ushered in a new age of surgical intervention that saved a lot of lives and prolonged life in general. I know I'm not alone in having a relative that would have been dead twenty years ago if not for the ability to radically alter the mechanics of a malfunctioning heart.
We shouldn't cease to try to understand the world because of the risk new knowledge brings. Rather, we should also understand the risk and act to intercept and mitigate it. But don't condemn Alzheimer's patients to a slow twilight death because of the risk of strong AI; that's not an acceptable tradeoff.
This depends on the estimated probabilities of worst case scenario. I don't think you can just handwave it as "always worth it".
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?).