The Fairness Doctrine was a failure of a policy that was exploited by Republicans in order to present sides of subjects with little evidence as if they were equally valid. The Fairness Doctrine gave rise to concepts such as mainstream acceptance of creationism and denial of global warming. The Fairness Doctrine is antiquated in the age of the Internet, and would be largely irrelevant in any present form.
No, market forces are not insufficient for creating sound public discourse, because only market forces allow everyone to have a say. With a free market, each person can vote with their own choice. Allowing only a few to control the market assumes that a few people know better than many people. If these people have poor information, they hurt many people. If only a few people choose poorly on their own, they mostly hurt only themselves.