First off, the brain was not impossibly small - see my previous post. Their brains were perhaps small by dinosaurian standards, but not by reptilian standards. Further, just because one has a big body doesn’t mean one automatically needs a bigger brain. Most of the background processes are done without having to think about them, so it doesn’t hinder the beasts in the slightest that they don’t have huge mammal brains. At any rate, I doubt stegosaurs were all that much dumber than, say, a cow (which, from my experience, are completely vacant).
Second, the second junction is by no means limited to those “dumb ol’ dinosaurs” (as many people seem to think they were). All dinosaurs had them to various degrees, as do birds and mammals.
Granted, they still probably wouldn’t make good riding beasts, but it is a myth that dinosaurs were dim-witted beasts doomed to extinction by their own stupidity. If anything, we get that distinction.