If you can raise the floor for everyone, so that we're all just better, what's not to like about giving everybody that treatment?
The same that's not to like about forcing anything on someone against their will because despite their protestations you believe it's in their own best interests. You can justify an awful lot of evil with that line of argument.
Part of the problem is that reality tends not to be as simple as most thought experiments. The premise here is that you have some magic treatment that everyone can be 100% certain is safe and effective. That kind of situation does not arise in the real world. It takes a generally unjustifiable certainty in the correctness of your own beliefs to force something on someone else against their wishes because you think it is in their best interests.
I might be wrong in my beliefs about their best interests, but that is a separate issue.
Given the assumption that undergoing the treatment is in everyone's best interests, wouldn't it be rational to forgo autonomous choice? Can we agree on that it would be?
In this video, Julian Savulescu from the Uehiro centre for Practical Ethics argues that human beings are "Unfit for the future" - that radical technological advance, liberal democracy and human nature will combine to make the 21st century the century of global catastropes, perpetrated by terrorists and psychopaths, with tools such as engineered viruses. He goes on to argue that enhanced intelligence and a reduced urge to violence and defection in large commons problems could be achieved using science, and may be a way out for humanity.
Skip to 1:30 to avoid the tedious introduction
Genetically enhance humanity or face extinction - PART 1 from Ethics of the New Biosciences on Vimeo.
Genetically enhance humanity or face extinction - PART 2 from Ethics of the New Biosciences on Vimeo.
Well, I have already said something rather like this. Perhaps this really is a good idea, more important, even, than coding a friendly AI? AI timelines where super-smart AI doesn't get invented until 2060+ would leave enough room for human intelligence enhancement to happen and have an effect. When I collected some SIAI volunteers' opinions on this, most thought that there was a very significant chance that super-smart AI will arrive sooner than that, though.
A large portion of the video consists of pointing out the very strong scientific case that our behavior is a result of the way our brains are structured, and that this means that changes in our behavior are the result of changes in the way our brains are wired.