I believe it's a chicken and egg situation because the reason why usage of the Lightning network is low is because there aren't that many services and merchants have adopted the off-chain payment network. But if suddenly all casinos started accepting Lightning deposits and allow withdrawals through Lightning then I'm quite confident that Bitcoin users would start believing that it's a feasible payment method, and there's no need to convert their Bitcoins to altcoins too.
The advantage of using the Lightning Network is not just for cheaper and faster transactions. It's a good layer that adds privacy for your transactions too.
Besides business owners should not only think on the now but on what will come next, so if a casino decided to accept deposits and withdrawals with the LN and this became known then all of those that want to use it and want to gamble will have no other option but to use that casino, and by the time other casinos realized what is going on, it will take them some time to react and adopt it too, but by the time this happened any casino that did this will have a first-mover advantage over the rest, gaining a significant advantage.