You don't have evidence for that. You presume that less intelligent animals are easier to control, but that's not at all necessarily true. The smaller brain -> less intelligence isn't even something we know.
That doesn't mean that humans are dumber now, though, nor does it mean that dogs were bred to be dumb, nor does it mean that dumb animals are easier to control. Humans with intellectual disability can in fact be extremely hard to control in my understanding!
That wasn't the claim. The claim was about the effects of domestication on cognition and at least for dogs it is very obvious they are less intelligent than their non-domesticated counterparts. Same is true for rabbits. So, at the very least, in at least 2 cases it is clear that domestication has lead to a decline in cognitive abilities. It doesn't take much to infer the same is true for people, domestication has reduced selective pressures on cognitive abilities.
If you know of cases and studies where the opposite is true then I'd like to see the research.
