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.
Not only that, they should have an internal swap or exchange. It's also a solution for most gamblers that find it hard to transfer and convert because they're too conservative about the fees.
They exchange internally with the altcoins of their choice and move it to their own wallets with cheap fee.