My opinion on what is a "quack" is weak!
By definition a quack can be described as......
"An untrained person who pretends to be a physician and dispenses medical advice and treatment".
Therefore someone who has had much experience in their chosen field cannot be described as a quack because they have acquired their knowledge and training through this experience and therefore can be described as "trained" in their specialty.
A certificate or degree only means they have reached a certain standard of this acquired knowledge through study, whether this be manually or academically.
I agree with you that a licence in a field is a form of control, so that true charlatans cannot be let loose on the public by giving ill-informed and inaccurate advice, but where this advice is based on a wealth of accurate knowledge then credentials are largely irrelevant.
For example, I offer advice on water-only-fasting based on what I know to be true and accurate information gleaned from the past 150 years, and do not see the need for credentials to rubber-stamp that accuracy.
Speaking of quacks, I suggest you read this article on iatrogenic deaths and adverse reactions.................