On the scale of Pepsi or Coke, a seemingly trivial amount like 1/2 a cent a can adds up to significant money. It’s amazing how companies pinch pennies when dealing in volume like that. They sold 32 billion cases of beverages in 2022. No idea what the real figure is, but let’s say 5,000,000,000 of those are diet drinks with aspartame… that’s 120 billion cans, so if the other sweetener cost only 1 cent more per can that’s 1.2 billion dollars.
Since the verdict on aspartame isn’t clear, they’d also have to tweak the formula for flavor, and switching would be somewhat of a PR admission that there’s something wrong with aspartame, I imagine they’re very reluctant to change anything.