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.