Will, there comes a time that all online gambling sites will be Cryptocurrency based?
No, fiat based ones would still exist no matter what because there would be always players who do prefer on playing fiat based rather than on considering crypto.
that is the sams as people using fiat over crypto. if there will be fiats , there will always be service or websites that accepts fiats. the only way to see all gambling sites will accept crypto is when crypto replace fiats and any other currencies but i dont think that it can happen so easily.
but besides , fiat gambling sites are not that popular when compare to crypto based gambling. the time will come that crypto gambling will grow more and fiat based gambling site will be totally dump out .