>Perhaps the issue of cognitive decline comes from sitting there vegetating rather applying themselves during all that additional spare time.

The push for these tools is to increase productivity. What spare time is there to be had if now you're expected to produce 2-3X the amount of code in the same time frame?

Also, I don't know if you've gotten outside of the software/tech bubble, but most people already spend 90% of their free time glued to a screen. I'd wager the majority of critical thinking people experience on a day to day basis is at work. Now that we may be automating that away, I bet you'll see many people cease to think deeply at all!