Is it one of those things like Saccharine, that might be cancerous if you ate 10lbs of it every day for 50 years?
With nearly every chemical, it's not as much the poison as it's the dose.
If diet coke caused cancer, guaranteed we'd know about it by now. A lot of diet coke drinkers are not casual, they're hardcore, and if there was a causality to cancer, it would be impossible to hide the correlation.
I do believe there are a lot of serious long term health issues that chemicals like artificial sweeteners contribute to, especially in America, but I'm not willing to hitch my horse to the carcinogen wagon to get them more attention.