"What if someone saw the gun, and went off the handle, and grabbed it?"
1) Of course! Completely normal, regular people frequently 'go off the handle' when they see a gun. The gun is what makes them crazy...

He mentions it happening in a courtroom... Where you already KNOW there's violent criminals coming into... Completely different scenario then a public meeting.
2) How is that the Gun's fault? Granted, sure, the guy could have carried it a little more securely, maybe at least level 2 retention.. but he is not required to by law.
3) Whats to say someone couldn't 'go off the handle' and illegally bring a concealed handgun of their own?
4) I'm sure the Secret Service had a scope on that gun the entire time. If some third party 'went off the handle' and grabbed it, and raised it in the president's direction, he would have gotten a .308 round in the chest before he could even get the sights up.