I have no doubt that they'll try to get away with as much as they can, but won't there be a a breaking point? When DCC was < 3% there was enough uncertainty to go mostly undetected. Before I was aware of what was happening I would think, "Maybe my bank wasn't using the most competitive exchange rate?" (I didn't know the rate was centralized with Visa and MasterCard.) The transaction value also might have also been low enough where the DCC markup in absolute terms amounts to < $0.50. (Think DCC at Burger King in Ireland.)

In the case of 10% DCC do you think people will still accept that, especially on larger purchases like hotel bills or nice restaurants? If the transaction posts for $100 more than your estimate for the exchange rate, that's well outside the margin of error. Even if unaware of DCC, do people really not check the exchange rate?