I don't believe there's anything whatsoever wrong, and certainly not health-damaging, with having a diet soda once in a while. Once in a while, being the key there.
2 liters a day? Sure I'd say not the best idea. Grabbing a Coke Zero when you're on the run and didn't bring a better option? Should be as worrisome as having to use the gym's adjustable incline bench instead of the fixed incline.
The info about artificial sweeteners eliciting an insulin response, I may be brosciencing but I'm not fully convinced that's a thing. I know that chewing gum and diet drinks have been staples of contest bodybuilders for a long time. If those things made fat loss more difficult, I think word would be much more widespread by now.
As for some artificial sweeteners being linked to cancer, pretty sure that gets back to being "dose dependent." 12-20 ounces of diet soda/Crystal light/whatever a few days a week is not the same as consuming mass quantities daily.