Fiat payment on line casino is not necessary, because most people don't use them for gambling, people prefer gambling off Fiat, they use other payment options via Crypto, haven't you heard that since crypto become a thing many numbers of new gamblers are now using crypto to gamble online, this has been ongoing since the last virus outbreak (covid19).
I have used some online casinos and the only payment options I choose are all crypto, Bitcoin, Litecoin, and Dogecoin, few don't even have any other payment options than crypto, so to me it doesn't matter, if you are planning to run an online casino, crypto payment is more than OK.
Those who already use crypto will recommend using crypto for gambling because we already feel the benefits. Payment using fiat may seem worrying because the bank may track the use of the money in the bank, so the bank will ask us about it.
And yes, just having crypto payment options is more than enough because nowadays, people are getting to know crypto better. But if a casino wants to add another payment gateway, that would be better because they can give their users a choice. After all, their users are not just crypto gamblers, as there are fiat gamblers turning to crypto casinos.