Gradual brain damage can strike at any time. You wouldn't even notice anything other than feeling more sluggish and having foggier thoughts. Imagine how many people don't even know they have sleep apnea that is slowly lowering their IQ. Imagine how many people have an oral herpes infection that reached their brain where it will now wage an endless battle to convert all of their neurons into derelict HSV-1 replicators. There could even be an evolutionary advantage to a brain automatically permanently entering a more dormant state after it senses it has reached the top of the pyramid, assuming tribes were less likely to self-destruct when members of the next generation had a fairer chance at usurping the throne.