IMO, the "rapid takeoff" idea should probably be seen as a fundraising ploy. It's big, scary, and it could conceivably happen - just the kind of thing for stimulating donations.
It seems that SIAI would have more effective methods for fundraising, e.g. simply capitalizing on "Rah Singularity!". I therefore find this objection somewhat implausible.
Many people complain that the Singularity Institute's "Big Scary Idea" (AGI leads to catastrophe by default) has not been argued for with the clarity of, say, Chalmers' argument for the singularity. The idea would be to make explicit what the premise-and-inference structure of the argument is, and then argue about the strength of those premises and inferences.
Here is one way you could construe one version of the argument for the Singularity Institute's "Big Scary Idea":
My questions are: