I think population growth also factors in. Population is leveling off. In the past century, the global population has quadrupled, so there are four times as many people to invent things in raw numbers alone. But global population will increase no more than 50% in the next century, which means we aren't creating a lot more inventors than we are now.
This is a good point, but also consider the proportion of the global population who have the opportunity to become inventors is hopefully going to grow over the next century. As a result, the absolute number of inventors may grow faster than by just growing the overall population size.
