I would hate to see "treatment" forced onto them because they're not as smart as we'd like.
If the analogy here is with depression, that doesn't seem a likely outcome. Depressed people don't normally have anything forced onto them, unless they make it clear that there's a substantial imminent risk that they'll actually kill themselves.
I think the things that will get a mental illness forcibly treated are (1) that it genuinely makes the person who has it unable to function independently, or (2) that it puts other people at substantial risk. Stupidity has to be really severe before it causes #1; I suppose the question is whether (in a hypothetical world where stupidity is medicalized and treatable) it would often be seen as causing #2.
I do, though, very much agree that the combination of giving "stupidity" a broad enough definition that it applies to a substantial fraction of the population and treating it as a disease seems really dangerous and open to abuse.
or (2) that it puts other people at substantial risk. ...I suppose the question is whether (in a hypothetical world where stupidity is medicalized and treatable) it would often be seen as causing #2.
Carlo Maria Cipolla's definition of stupidity is clearly relevant here. (Link is from the Less Wrong Wiki.)
It's great to make people more aware of bad mental habits and encourage better ones, as many people have done on LessWrong. The way we deal with weak thinking is, however, like how people dealt with depression before the development of effective anti-depressants:
The only "anti-stupidity drugs" we have are nootropics. But the nootropics we have weren't developed as nootropics. Piracetam was, I think, developed to treat seizures. L-DOPA was developed to treat Parkinson's. No one knows who started using ginkgo biloba or what they used it for; it was used to treat asthma 5000 years ago. Adderall derives from drugs used to keep soldiers awake in World War 2.
And none of them are very good against stupidity. AFAIK, to date, not one drug has been developed by understanding and targeting the causes of different types of stupidity. We have the tools to do this--we could, for instance, sequence a lot of peoples' DNA, give them all IQ tests, and do a genome-wide association study, as a start.
We don't research these things because society doesn't want to research them. People don't conceive of stupidity as a disease that can be cured. We need, somehow, to promote thinking of stupidity as a mental illness. As something drug companies could make billions of dollars off of.