People with "no" (very small) brains due to hydrocephalus have been discovered who are successful mathematics students and public servants. So, not sure the scientists are right here. As long as the brain is shrinking slow enough it may not be a huge deal in terms of mental ability.
http://mentalfloss.com/article/70204/man-without-brain
https://www.theguardian.com/education/2003/oct/02/research.h...