The problem is that artificial sweeteners create an insulin response even though they are calorie free.
The insulin causes two things: 1) it tells cells to uptake sugar from your blood, which leaves you slightly hypoglycemic, since the insulin response is out of proportion to the actual sugar load consumed (particularly on an empty stomach). 2) chronically elevated insulin leads to insulin resistance (the precursor to metabolic syndrome which makes you fat, diabetic, hypertensive, etc).
This is the real reason we need to stop using most artificial sweeteners. Stevia and Erythritol have not been shown to cause this insulin response. It doesn't mean they aren't also bad. Only that for now, the jury is still out and they appear to be safe. Stevia in particular has been associated with something of an opposite effect, where it seems to improve insulin response in people who consume it.
Now for the popular reason they're getting rid of it:
Aspartame itself appears to have neurological effects as well, which in sufficient quantities causes problems. I personally know that any more than 20 oz of Diet Coke starts making me feel "odd" for lack of a better way to put it. It's not the caffeine. I don't get the effect from non-aspartame caffeinated drinks.
This seems like a relatively minor reason to stop using aspartame unless you're consuming vast quantities. Regardless, people think it's a neurotoxin and can't have that. (Forget about all the other benzene additives, colorants... even caffeine itself is a toxin).
Anyway, glad to see they are doing away with it. Here's hoping they don't use use Sucralose, which is even worse than Aspartame at producing a phantom insulin spike. (And people get upset at the chlorine... but say nothing about drinking chlorinated water or soaking in hot tubs).