We agree on the rest, but I'd like to see evidence for your claim, which I disagree with. Not that the coverage has changed - it has, for the reasons I listed - but that the Fairness Doctrine specifically had the very large effect you claim.
Start with Edward R. Murrow and work forward. Ed Murrow was pretty much one of the driving forces behind the Fairness Doctrine, which by the way was also spurred by our past experiences with the "Murdochs" of the preceding era, in which television did not yet exist.
The great magnates like W.R. Hearst were also partly responsible. The FCC was tasked with helping to ensure that television news and public affairs would not fall victim to the same political ambitions of another Hearst style magnate who could manipulate the will of the people for his own agenda.
The solution was quite simple, in that the Communications Act of 1934 required that broadcasting on the public airwaves center around "service in the public interest" and therefore transmitter licenses would only be granted IF stations SERVED "in the public interest" with regard to what the FCC termed "news and public affairs".
The Fairness Doctrine was entirely a creature of the new television era itself.
Thus, if one asks what evidence there is, I would have to point you to every bit of news ever created for television prior to the early 1990's.
Instead of asking for evidence that the Fairness Doctrine prevailed, perhaps you should ask if there is evidence that the lack of a fairness doctrine has allowed evidence of overt and covert bias to sink into the news we consume today.
By the way, grant of license was the only "enforcement" that the FCC had for the doctrine.
There was no schedule of fines or other punishment. License holders simply were required to demonstrate that they adequately served the public interest and offered enough time to opposing views on editorial content.
The way station and network owners complied was actually through an informal gentleman's agreement to operate their news departments as loss leaders, and thus they were able to insulate news from the effects of chasing profit.