First a lab leaked virus, then one man with a giant ego and a nuclear button.
As tech becomes more powerful, it becomes easier and easier for one accident or deranged person to kill us all.
Your figure of 1/3 deaths, or up to 3 generations to rebound might be too optimistic. We just don't know.
Given how reliant we are on existing supply chains, their complete breakdown could send us back into the stone age. We've certainly seen how even a relatively mild pandemic can disrupt our lives for years. The effects of something far worse would be difficult to predict. Us measly tax payers can only hope for the best.
That news to the ancient egyptian civilization, mesopotamian civilization, incan civilization, aztec civilization, etc.
I can recommend the writings of William J. Perry [1] and for blunt emotional effect the BBC movie "Threads" [2].
Either way, 800k years feels very optimistic right now. We need to stop thinking about a trillion potential future people over a million years and start making things better for the few billion or so who will live in the next 20-100 years if those trillions are ever going to exist. We're really really bad at this near-medium horizon, endlessly shuffling the costs of our choices onto them and their children as if we can amortize it forever.