Their only answer is "if we invent AGI and fire everyone, the profits will be limitless".
"If we end up misspending a couple of hundred billion dollars, I think that is going to be very unfortunate, obviously. But I actually think the risk is higher on the other side . If you build too slowly and superintelligence is possible in 3 years, but you built it out assuming it is possible in 5 years, then you are out of position on the most important technology."
His assumption is that superinteligence is close, its just a question of whether it is 3 or 5 years!
This made me think Zuck sees it as a question of when rather than if. I.e its more a question of 3 vs 5 years rather than possible vs non possible.