"Smarter" is difficult to parse this way. We stand on the shoulders of giants, each subsequent generation benefiting from the store of knowledge of our ancestors. We are "smarter". Biologically we progress (evolve) into our environment. That can go either way for "smart".
I agree that we stand on the shoulders of giants TBR. In a way, the work of the great thinkers before us has the potential to enable future generations to become lazy and unable to think for themselves - they don't have to when the hard work has been done for them. There seems to be the potential for humans to become more stupid over time if we no longer have to think for ourselves.
"There seems to be the potential for humans to become more stupid over time" - Yes. Due to the forces of evolution, given proper conditions, we could become "less smart". If additional "smart" is unnecessary for our survival, there is no reason for that to not be true.
It is very easy to see Idiocracy (one of my favorite movies) as a documentary. It might take a little more time then that, but concept is sound. Yes, as a stereotype that works, more "intelligent" people are having less children later in life. This leads to generational compression multiplying the issue (if there is one).