There is more weight loss to be had switching from Diet Coke to water. It's obviously not the calorie content, but the insulin response from sweet tasting things that will occur even with artificial sweeteners.
I've been using erythritol and stevia as sweeteners for years, and even with those, I find myself craving sweet things after consuming them more than when I go relatively long periods of time without eating sugary things or using sweetener.
The end result, at least for me, is that I crave sweet food more even when I use artificial sweeteners, making it harder to say no to eating shitty food later on.