Fungo,
I think you did the right thing in telling them the truth. When they made it clear they didn't want to listen you backed off. I don't think you have any obligation to do any more.
If you do decide to pursue the matter you'll just have to couch things in terms of "making a great swing even better" if you hope to have them listen.
Our problem tends to be almost the opposite. For example last night during my son's hitting lesson he got a few compliments from onlookers about his swing. He was having a good lesson because he was getting away from his main swing flaw on many of his swings but his instructor and I both feel that he's just starting to make progress and that we can expect a lot of backtracking before he gets to the point where he swings "right" naturally and going back to the old swing flaw is the exception.