People with Down syndrome are generally very happy, what's about inducing it?

Don't quite follow - you mean, 'Would it be ethical to induce Down syndrome, given that people with Down syndrome are often very happy?'

Well, maybe. On the other hand, my impression is that as much as caregivers may want to deny it, a Down child imposes major costs on everyone around them. Inducing high IQ would not be obviously worse even in the cases where they flame out, would be a lot cheaper, and would pay for itself in inventions and that sort of thing. So there are lots of better alternatives to Down's, and given a limited population, the optimal number of Down syndrome may be zero.