"Legal mind-enhancing drugs are a great danger to society. They will put everyone in the position where they have to take those drugs, or risk perpetual unemployment."
Replace "drugs" with "education", and ask what should be done. Ban learning?
By contrast to drugs, "education" has thousands of years to prove it's long-term medical safety. Different drugs have proven to have all kinds of nasty side-effects to a point where it is normal to hold them all suspect by default.
Sometimes you have to take risks. There is some question as to whether cell phones have long term medical risks, but I bet you use a cell phone (for increased productivity no less).
Replace "drugs" with "education", and ask what should be done. Ban learning?