When the business is getting paid in bitcoin, pays in bitcoin, and works only with bitcoin and its revenue is not dependent on bitcoin price rise/fall, it makes no sense to measure things in fiat terms.

I've made this argument elsewhere about gambling sites but it also works with mixers and any other business that is always working purely with bitcoin, maybe more so for mixers since their customer count increases unlike gambling sites. Kinda like this forum's donator status still costing 10BTC.

With that said, I wouldn't be surprised if the rates were lowered at some point since we are still living in a fiat world where 1BTC is not 1BTC but is X$.