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.
Agreed, most gamblers prefer reputable gambling casinos as they know how much their funds are protected no matter how much the fees are. Also, no matter how low the transaction fee on newer gambling sites as long as they aren't reputable, gamblers would disregard them especially High Rollers.
Also, there is also a reason why older casino still runs up until now no matter fewer changes and updates are done on their site as they have already built their Reputation around the gambling community. An example of a great gambling site is Primedice which has been on the gambling industry for quite some time now.