I disagree. Someone's expertise is based on their education and their experience. It has nothing to do with whether you accept it or not.
If you are in sudden need of someone to land the 737 you are flying on, you should look for an expert on flying planes. No matter how much expertise I claim, or how much you believe in me, I am a poor choice to land that plane. I have never been to flight school and have zero experience.
The proper question in this case is "Who here has a jet pilot's license?". Once you find that person, every one else should either follow their instructions or sit down and shut up. That person is the expert.
If you believe that there is some jet pilot's conspiracy to keep dentists and bakers from expressing their opinion on the right way to land a 737, then you have a problem. That dentist that you decide to listen to (rather than the pilot) is going to get you killed.
You get a pilot's license by going to flight school, studying how to fly a plane, and working under people who are experts before you. Does having a jet pilot's license mean that you know more about flying airplanes than other people?
Well... yes it does.