Is this just called dark chocolate? Kidding aside, I'm hoping they don't replace it with some sort of chemical sweetener, I'm always suspicious of sugar replacements.
you realize every sweetener is a chemical, right? What if they replace sugar with sugar? There are tons of types of sugars that can replace one another. Lots of sweeteners actually come from plants and microbes.
It's not a replacement. The 60% remaining is just tastier to your mouth.