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The irreversible global catastrophe is imperialist militaries massacring people with little risk to themselves. 
The irreversible global catastrophe is assembly line robots at the Toyota plant becoming sentient and going on a freaking epic terminator style mass shooting and enslave everyone except me 
The problem of "how could AGI take over the world" is the same thing as "how could a billionaire take over the world". To affect the physical world they just pay people to do things, no trickery required. If the factory needs to keep the lights on, a private army will guard it. Runaway AGI is a problem of capitalism (and its prominence in discourse is a reflection of people's fear of capitalism). It's just "what if corporations were smart".