PBS, being funded by taxpayers, does not have to rely on ratings alone to survive. They'll get gov't money either way. This leads to the programming only being affected by the judgement and credibility of those in charge.
Its funny, really. The only news station that shouldn't have an authority to exist is the only one that is fulfilling its job as a news organization. PBS news is extremely fair, comprehensive, and accurate. Meanwhile, the other privately owned news organizations that should be totally acceptable are doing a pathetic job of being honest informative to their viewers.
Honestly, for right now, I'd be happy to continue to fund PBS as its the only news station out there that won't talk about flag pins for a week straight. I think getting rid of one of the few decent mainstream news organizations would be a mistake at this point in our history.