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.
For us we should try to avoid overly involved with how casinos run their business, I am sure that most gamblers will want to be on the winnings side or at least minimize their loses but in the end, we should know that if we want to have a gambling platform to at least play on, then we have to support those casino's to at least sustain their operation.
So we shouldn't propose what we know can kill the casino as a business, we must apply every possible caution when we are making such suggestions.
Lolz, you are very funny bud, online gambling casinos make more then enough money to sustain the business and keep it running as long as long as possible, don't be ridiculous by thinking that the idea suggested here by the op is one capable of ruining a casino and making them go bankrupt, it never will happen and I tell you why.
Like I think I've explained before, if a casino decided to implement the proposed idea, understand that gamblers in their greed, more than half gamblers may never even bother making us of this feature, either because they don't even know it exists, or because they simply will rather either lose everything or Win the maximum potential winnings, no in-between, so in the end, only a very limited number of gamblers will end up always using this feature to better their chances of walking away with something in every bet, and know it that this isn't even a guarantee because not all matches to get a cash out offer.