Comment by rictic

The author has written about stacked sigmoids in the past, e.g. in https://slatestarcodex.com/2018/10/15/is-science-slowing-dow...

I suspect that he started down the path of considering them more deeply here but found that they didn't add much to the analysis. A stack of sigmoids ultimately either gives you a single sigmoid if you run out of new innovations, or an exponential if you don't.