It will eventually come through. There has to be only one miner that doesn't require a high fee to mine a block and to add the transaction. Then it can be confirmed and we will pay you instantly.
You still have the duty to place a higher fee than keep your players waiting for their withdrawals to confirm. That's just unethical. If you also put a low fee, there is a chance that their transaction does not go through and it is dumped from the memory pools of the miners hence canceling the transactions.
Being cancelled isnt possible but as expected transaction confirmation would really be long specially to these days which unconfirmed transactions is keeping on piling up and as adviced we should really put up high tx fee and its really a sacrifice if you do really care for your sites reputation.Instant withdrawals or deposits is a great feature but sometimes blockchain do really make mess.