You offer as examples, media organizations which are difficult to fire for any single topic. BBC has broadcast shows both for and against the OCT. Popular Mechanics published an article defending the OCT. South Park is not a media source like the others you list. From viewing several of their videos, they seem to be sometimes for and sometimes ridiculing overzealous Truthers.
Kevin Barrett has been hurt from his own experience of being fired for talking about 9/11 in his classes, and makes an emotional over statement. But, Geraldo Rivera and Judge Napolitano are doing just fine, even though they have publicly stated there is something very odd about Building 7.
You are correct in calling Kevin out on his hyperbole. He should have said that questioning the Official Conspiracy Theory MAY have a negative impact on one's career. Not that "If A, then B". Questioning 9/11 has decreased my income as a US Government contractor. But there are things in life that are more important than money.
It would be so easy to stop questioning 9/11, close my eyes to the clues of an extremely clever crime, and "go along" with the "normal" crowd. Many JREFers would call me a "smart guy" if I agreed with the OCT. But inside, I would feel bad for letting the murderers of 3000 people get away with it. Truthers are just asking for a standard criminal investigation, where commissioners don't quit, calling it a farce. If it turns out the FBI has enough evidence to formally charge OBL, then great.
Speaking truth against the majority takes sacrifice, but is worth it. People question 9/11, out of a deep caring for truth, justice, and making the American way good again.