Again I'm not seeing the logic at all.
If you needed serious surgery and Bob down the pub said he'd do it with a carpet cutter you'd be OK with that if every medical professional said that Bob was a really bad choice?
Different scenario. In your example, there is reason to believe that every medical professional has knowledge of Bob's skill and can properly evaluate his skill. Established media and political insiders are afraid of Trump because he (correctly) dismisses them as mostly useless idiots. That frightens them more than any political opinion or position on actual issues. It's the same reason many in the GOP were/are terrified of Trump, they are terrified of anyone that can't be easily brought to heel and tow the establishment line. It's obvious that those that favor entrenched establishment in the media and politics (regardless of party) are going to be against someone that dismisses established media and politicians.