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.
There are still gamblers who lose though, so casinos can still earn decently. Also there are no guarantees that those who cash-out their money will never come back again because I'm sure many of them will re-deposit again eventually and they can even lose more money than compared to their winnings.
If we are talking about the cash-out option in sports betting, then the cash-out for them is already small. That is for the gambler to think twice if they will cash-out or continue playing but I think many still continues because of that fact. Some can win and can be more happier, while some lose and regret their previous decisions pretty badly.