Yes, new gambling sites can add altcoins withdrawal system. By following this, it may help them for marketing. It will be considered as a marketing tool for them which will help a lot to grow faster and also will he helpful for gamblers too. But we are doing withdraw with forcibly high withdrawal fee.
Even with that feature, those new gambling sites will take a long road before being recognized. In some cases, a gambler will still stick to their preferred gambling site disregarding fees as it's riskier to just try those new sites just because they support all coins.
But in the event that the site survived the hard part, surely it will gather more gamblers on their site. Not that easy though if we will look at how vast now the crypto-gambling world is.
That is somehow true. Gamblers will try new casinos, but they will only spend small amount of funds just to try out. But when it comes to high amount of deposits, they always go back to their old casinos. Because they are confident that they will not lose their funds for nothing. But yes, if these new casinos survive, then that's great. And it would take time to gain trust from the community. They really do need to work on it also.