Something big enough to kill the dinosaurs could kick us into a new ice age. There'd be firestorms first, supervolcanoes. Any buried radioactive substance would get kicked up into the atmosphere and be trapped by the remaining debris. Voila, everything's both dead and radioactive. I'm sure it wouldn't be great for nuclear power plants or active weapons either.
I'm not saying it's more likely than people being idiots, but it might be an alternate possibility.
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I'm not saying it's more likely than people being idiots, but it might be an alternate possibility.