People come up with a lot of rationalizations for being opposed to artificial self-enhancement. But most don't apply the same standards to "natural" self-enhancement, even when we
know that there are severe tradeoffs.
In particular, people overly discount the value of life-seconds. What is the opportunity cost of putting in however many extra hours it would take to practicing chess, in order to account for the benefit of using modafinil? Is that cost outweighed by the small risk that there's some hidden deleterious effect of modafinil which has somehow eluded scientists for thirty years? And even if so, why should we police individual choices of what risks to take in life?
After all, we know that baseball pitchers are at extremely high risk for shoulder injuries. Should we enforce rules that limit practice time, intensity, and technique, so that other pitchers don't have to risk their bodies to compete? Should we put an asterisk next to the names of pitchers who are found to violate those rules, and kick them out of the sport?
That seems, broadly speaking, not to be the kind of choice we've made as a society when it comes to "old-fashioned" self enhancement. Yet we do make that choice when it comes to "artificial" enhancement. I put it to you that the distinction is arbitrary.
Edit: Just one other thing: "mind-altering substance the users of which appear to experience no negative effects and major improvements in their functioning"
That very nearly describes modafinil for most people. (If you're going to say "all people", you've set the bar so high that no drug will ever reach it. Ibuprofen doesn't even meet that bar.)