What I think what most people count on is the inanely fast ACCELERATION of technological progress. The amount of advancement between 1930 and 2015 will be orders of magnitude less than 2015-2100.
Why do you say that? I think the technological progress will continue to grow exponentially. There are more researchers now than at any point in human history. The economic incentives are still at least as big (relatively speaking).
At the moment there are many industries with great potential for applying machine learning and various other automation techniques.
And considering that many leading AI scientists are expecting smarter-than-human AI within this century (or the start of the next one), I certainly fail to see your point.
Anyway, there is still a lot of room for exponential growth of human knowledge, in a lot of fields.
