Over-insurance
First, the casino edge on insurance is not 68%. Off the top, it is about 1/13, which is less than 8%.
Second, if over-insurance were allowed, I don�t know that ploppies would take it. I think you would have a much higher percentage of card-counters taking it than ploppies.
I think you are right that for conventional insurance, there are a lot more ploppies who take it than card-counters. Were it the other way, I think the casino would quickly move to eliminate insurances.
However, card-counters would definitely take the over-insurance, but I think the ploppies would still insure.
Actually, I think most of the insurance money the house makes on insurance if from �even money�. I see a lot of civilians take even-money; I see few of them take insurance.