Andrew Ng, Yann LeCun, and many other people who have ACTUALLY worked in AI are the ones who scoff at it. They don't need to make arguments because what do you respond to a young earth creationist?
All of the arguments in Supreintelligence or otherwise are simply that AI will eventually exist. The only argument for that it will come by 2050 is a badly conducted survey of non-experts.
Should we worry about all sorts of existential risk which could arrive in any undetermined time in the future?
The whole project is so absurd, it's hard even to begin to make any counter arguments, because none of the arguments make any sense.