But they are not forced to use bitcoin in gambling, most gambling sites now have added a lot of altcoins already as they have already anticipated that fees will increase and gamblers will have to pay for a high fee which is not helping them. Gamblers have to be smart, use other alternatives in order to save from fees, besides, I believe that everyone knows how to trade, then if we have BTC, we just simply trade it to altcoins at our choice then we are good to go.
You're right, some websites offer altcoins, but many still have just BTC and ETH. Sure thing, a gambler can switch to another one for the sake of saving money, but sometimes it's not an easy thing to do, when you're already used to a certain online casino, have your gambling mated in there, aware of all the features and policies.
Besides, that sounds kind of strange for gamblers to worry about high transaction fees. I see numerous players placing bets over hundreds of dollars each day, I guess paying $10 for a transaction won't be much of a problem for them.