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I still think that "lifeless rock" does not specify how lifeless - theoretically extinct or just lifeless enough to make human life either extinct or just miserable. I took it as the latter, and in that case even lesser cases than 75% of all species would suffice.
The first case, the theoretical and non-human focused pov is quite another thing. Like you said, there's so many opportunities and adaptations for life to seap through the combs of doom :)
Going a couple comments up the chain:
So I took it to mean all life on Earth being dead. As long as one microbe survives to reproduce and start evolving it doesn't count.