In my completely uneducated-in-this-field view, "brain activity" can be broken down into two distinct types - the first type involves the raw procedural work required to solve a problem like 3012 x 42 in one's head, and the second involves higher level conceptual, critical and creative thinking required to say write a book or design a complex algorithm.

In the past, computers, calculators etc. have reduced the need for type 1 thinking, but I believe LLMs are increasingly replacing (or emulating / mimicking) type 2 thinking which - again entirely in my view - is more harmful to individuals and society as a whole.