"stops radio and TV from filtering":
I think radio & TV are still trying to maximise their audiences, but with so many different options for the consumers, they have to target their output to specific (consumer) groups. The media and the record companies are filtering like they used to do, but the criteria have changed / had to change because the circumstances have changed: In the 70s and 80s Decca & the BBC turned to JK (who gave the world Genesis, the Bay City Rollers & 10cc), nowadays Sony Music & ITV employ Simon Cowell (who will be remembered for Susan Boyle and not much else).
"discourages mass appeal":
Mass appeal is still the goal (see the media strategies Elberse describes/analyses in "Blockbuster"), but the meaning of mass appeal has changed: today "mass" means sales of about 10 to 20 per cent of the numbers reached in the 90s.
"deters gradual build of talent":
Yes. But long term thinking only makes sense if the circumstances stay more or less the same over time. If there is a certain standard / a common quality (shared by producers & consumers) which is the basis for success. That was the case with rock music: Labels, musicians & fans shared the same belief (the better you are, the greater your success). In this system (which Simon Frith called "The Rock" because of the large base and the small top only reached by very few artists) gatekeepers have the function of quality control, to make sure that the acts that "deserve" it rise to the top (because of the great "art" they are creating).
Today "The Rock" is no longer working. Since the 80s there is the "Talent Pool" (another model by Simon Frith): The media & the reccord industry are fishing in the "Talent Pool". They don't plan long term, have no standards or quality control. They are in the business of making money (not "art") and do whatever is necessary to get a hit today. The "artists" and their music don't really matter.
Here is a very useful analysis of the long-tail-concept:
www.hypebot.com/hypebot/2013/10/the-man-...d-the-long-tail.html