So where was this drop in activity? Wouldn't be everywhere in the brain - probably the areas required for the task AI was doing. If I have AI write redundant code, the right side of my brain should not be affected. If it draws a picture, I should still be as sharp in math as I am now. Some skills are no longer needed.
I was talking about this in last paragraph.Finally, nothing is rotting - we are not losing brain ability like in a stroke. Everything that you lose because of a tool can be relearned if you lose that tool. Some things are not worth worrying about. 
Who is worrying? "Rotting" is a hyperbole, but using meme terminology is fitting here. What LLMs are doing aren't in comparable to wikipedia or your mom's washing machine.
What i am talking about is creativity, which is basically problem solving that you need in coding as well. And sure, you can relearn skills, but if you stop using for example your legs for significant amount of time, you lose muscle, and if there comes a time when you need to walk without help, you might not have that half a year to train yourself back to shape because you needed that ability now. So why wouldn't you keep that skill up now?
We are talking about brain connections. You don't learn your skills back in a week.