Thanks, A. Great thoughts.
I don’t mind a little cockney, but I wouldn’t want to read a whole novel of it. In my mind, a main character should be easy to understand. Even if it is not totally authentic to the character’s region or culture. If a main character of mine spoke “cockney” or “cajun” or another “difficult” accent, I would only add an occasional word or phrase, I wouldn’t try to write entire dialog that way. Actually, I wouldn't make the main character speak cockney or cajun in the first place. That’s too much work. LOL!
Of course, I am talking about fiction. I think is fine to trade off some authenticity for readablity.
I find the same issue with technical terms in science fiction. When a writer overloads the book with tons of new terms for technology and locations and new species, it can be too much for me. I don’t want to struggle to understand what they are writing about. A few new terms are fine but don’t overwhelm me. And make it obvious what you are talking about when you use new terms.
It is a balancing act.