On the one hand, everybody would end up healthier, smarter, more productive, more attractive, richer (less healthcare spending), more psychologically healthy (the gut is the 2nd brain, bad diet means bad emotional health), will live longer, have their friends and family live longer.
On the other hand, people will be unable to satisfy their sugar addiction (sugar is completely unnecessary from a nutritional point of view, and harmful even at low doses).
Nothing comes "free". If there is an alcoholic in the family, that lowers the whole family's nurturance level, degrades the marriage, increases the probability of divorce, and undeniably hurts the kids. Sugar addiction is no different from alcohol addiction. The addict isn't the only one who suffers. Similarly, if your social circle is fat, you're more likely to be, and stay, fat. Getting rid of junk food has no downsides, unlike getting rid of alcohol. Junk food is harmful even in low doses, whereas alcohol isn't in moderation.