And just who gets to decide what constitutes "The Public Good?"
The bottom line is the only criteria there should be for a broadcaster to stay in business is if they can make a profit.
And I guarantee in a true market, broadcasters who engage in filth for the most part would go out of business.
Unfortunately many of these channels are part of larger broadcasting conglomerates, like Viacom, who are subsidized. Also allowing cable companies the option to not have to go "a la carte" with channels would be a big help.