Of course this is just my opinion, there is nothing scientific about it, but...
I have been a diabetic for 27 years now, and have been drinking diet soda/eating products with artificial sweetener for the same amount of time. I have been filling my body with aspartame/splenda, and so have the hundreds of diabetics I have known over the years.
In that amount of time, I have known maybe five people who have died - all died of diabetes complications. I have yet to hear of anyone getting sick/cancer/whatever and have it blamed on the artificial sweeteners we have been ingesting for years.
Not that it doesn't mean that I couldn't wind up with an aspartame-related health issue 40 years from now, but honestly, if that happens when I'm 70 from something I've been doing for 65 years, I'll be okay with that. :)
I'm just saying, I've been surrounded by a population that consumes the stuff in great quantities - some people I know drink 4L of diet soda a day - and so far, nothing.