It is, but why is that a problem? Moving from physical labor on the farm, factory or construction site to sedentary work and lifestyles would also atrophy our muscles so now we pursue physical activities we actually enjoy like gym and sports instead of hanging on to those dangerous and strenuous physical labor jobs just to stay in shape.
Similarly, you will use the AI to churn through the repetitive mundane tasks the AI can do, and then work your brain on the activities you actually enjoy.
Now if you choose to not exercise your brain and dumb yourself down the same way sedentary people don't exercise their bodies and get obese, then I guess that's on you, but we can't force the world to technologically regress just because some people are lazy and don't exercise, the same way we haven't forced construction workers to use pickaxes instead of machinery to tear up a road just to make sure they stay in shape and don't get too fat from sitting in their air conditioned excavators.