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.
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.