Who decides what is a net benefit to society?

In capitalism people vote with their own money: if there is a demand for some good or service it keeps getting provided.

Of course you might argue, that average person can be manipulated into buying things that are harmful to them, but if you want to prevent that there's really no way to do that efficiently other than turning the whole country into a totalitarian surveillance state. And you also need an elite, a group of people who are above the law because they are just smarter, and know better and therefore set the rules for lesser beings.