So, there was a
study (not yet peer reviewed), and results probably surprises anyone.
Group's brains were measured with a method known as Dynamic Directed Transfer Function (dDTF), that basically measures connections on the brain.
It was measured on using LLMs during tasks, using search engines, and using just their own skills unaided.
And compared to baseline (not surprisingly) LLM group had significant drop in brain activity.
My take from this is that, in a long run, if unaided group would keep using their brains for tasks, they would develop their brain because of training, while AI group would eventually lose even their existing skills (at least when it comes to writing essays). Because if you don't use your brains, those connections on the brains wouldn't be needed either.

There are ways to use AI that benefits people, but using them for your creative work is just killing your brain.