We may well get some smooth-talking snake-oil salesmen, but I think you underestimate the ability of people in small groups to identify and weed out such specimens. If some squeeze through, they'll be less effective when surrounded by bright people who are not easily swayed by a glib tongue.
I've seen Codes of Ethics for most of my life. We just had a suggestion for a new code of ethics in my state ... proposed by a team operating under a franchise created in, I believe, 1978. Their past efforts to foster ethical behaviour have failed and now they are making a new attempt. The new code was proposed on virtually the same day that someone who worked on the text of a new law to stop "Pay for Play" in our state wrote a memo to one of the political parties explaining how to evade the very law he'd helped draft.
The problem is, in part, that the people responsible for enforcing the ethical behavior are the same people who write the loopholes that subvert it.
The solution needs to be more fundamental than that.
Fred