I agree, personally I think POTUS and other government officials should even be banned (not by twitter but by law) to officially uses such private forums.
Nah that would put incumbents at a disadvantage against their competition.
And, forcing them to use more official accounts like @POTUS might give existing office-holders undue advantage.
I think drawing the line at inciting violence is proper. We do need more competition at the platform level and I think this comes when we establish a competitive environment among ISPs and content conglomerates by reinstating net neutrality and discouraging regional broadband monopolies.