Our brain is not the only matter that might make us fail. There is the overpopulation of our species. That is a cause for concern as well. Also in evolution, survival of the species is success. But can we consider mere survival a success? Sure we will evolve over time, but what if we were to evolve smaller brain sizes as we progress further along into future species? That may have occurred before in our Genus in a small population species "Flores Man" (so named for the island in Indonesia). Would that be success for humankind? I would say not. So even evolution becomes a threat to "humans."
I found a moral in these lines of thinking. Evolution can be scary if taken as a whatif with respect to the future.