With this your suggestion, the casino will be at the losing side whenever the gambler set the cashout. And the bookmarkers will look for alternatives way to bypass that feature. Either they will make the cashout very small or remove the cashout features in most of the game and only in specific games which they know that the percentage of cashout will be minimal. Though the proposed suggestion is good but I don't think casino bookmakers will implement it because they are also using that to make money.