It's hard to tell the difference between the very first little bit of an exponential curve and a linear one.
And a logistic curve is even harder before it starts slowing. I think the various arguments for superhuman AI are far more significant than the projected curves.
Here's a great article by Paul Allen about why the singularity won't happen anytime soon. Basically a lot of the things we do are just not amenable to awesome looking exponential graphs.