If you knew AI to be radically more transformative than other technologies, I agree that predictions based straightforwardly on history would be inaccurate. If you are unsure how transformative AI will be though, it seems to me to be helpful to look at how often other technologies have made a big difference, and how much of a difference they have made. I suspect many technologies would seem transformative ahead of time - e.g. writing, but seem to have made little difference to economic growth.

Here is another way of looking at things:

1. From the inside it looks like automating the process of automation could lead to explosive growth.
2. Many simple endogenous growth models, if taken seriously, tend to predict explosive growth at finite time. (Including the simplest ones.)
3. A straightforward extrapolation of historical growth suggests explosive growth in the 21st century (depending on whether you read the great stagnation as a permanent change or a temporary fluctuation).

You might object to any one of those lines of arguments on their own, but taken together the story seems compelling to me (at least if one wants to argue "We should take seriously the possibility of explosive growth.")