Well, like most people, I have never listened to Limbaugh,
and consider him to be a "fringe" pundit, as well. He's on lots of stations, of course, which shows a good understanding of talk radio and what drives it, but he says nothing of any value, so...
Progressive radio has not done well, really. That's probably because the target audience for it doesn't really exist. There are many progressives, but not so many radio listener among them, I think. The genre simply never generated enough listeners to get advertisers to buy time, so there was no economic basis for progressive radio, really.
Radio talk no longer seems to me to be a very viable means of selling ideas. I don't know, but it sure hasn't worked for the progressive movement nor even the liberal movement. I don't see any path toward making it viable, either.