Yes, suggest them to convert their funds as I also suggested some of my friends to convert their Btc or Eth to altcoins. The fee is limited, so it is not a big deal for big amount gamblers. I know some people who even deposit such amount to play will be considered as fee for others too. So they won't use Btc or Eth. The easy way is for them to convert.
We can suggest the new gambling site add altcoins to their site to help the gambler reduce the withdrawal fee if they use bitcoin or ethereum. With the increased price now, people feel not right with the fee, but they can not do anything except follow it.
Nowadays it shouldn't just about the coin, but about the provider as well. if a website can have few options like binance chain, ERC-20 i think it should be better
It needs more time to have another option such as adding binance chain or else, but the site can add the altcoin first in their site. But in the future, they can try to add the other option as the payment system to make the members like with the alternative coins.