I think you are missing the vital point. Humans don't need to become smarter as individuals. We create societies, which are complex human systems of knowledge-transfer and development. When we are many, 1/100.000 will create the microchip or the lithium battery. More importantly we grow in knowledge for every generation by being able to speak, while other mammals (who also learn) can't pass knowledge on as effectively. That is our edge. Therefore I think that any question of our intellect and its prospects will likely be answered by technology, as it has indeed always been. The phone today is one thing, but don't tell me in 100 years implants won't be accessible and in widespread usage.

"Collective Learning" is what I heard it named. Since that is how we conquered the Earth.

Try 288,000 years. Homo Sapians have had the same brain size since then, probably earlier.