you are obviously not past 50 years old. At that age, eventhough I "train" (as you imply it) a lot (I code about 50 hours a week, 40 at work, the rest for my own projects; not of them easy, my job is writing numerical code for simulation and my pet project is writinga very accurate emulator (which implies physics modelling, tons of research, etc)). I can definitely feel I'm not as productive as before (before I could sustain 60 hours a week) eventhough I do feel I'm using my brain to the maximum...

So yeah, past a certain age, you'll be happy to reduce your mental load. No question about it. And I feel quite relieved when Claud writes this classic algorith I have understood long ago and don't want to re-activate in my brain. And I feel quite disappointed when Claude misses the point and I have to code review it...