The very big name podcasters don't need radio. And they'd probably be stupid to do it. Let us count the reasons. First of all, they make more money online. Second, they don't have to wake up in the middle of the night to do a morning show five days a week. Third, they don't have penny-pincher bosses breathing down their necks. Fourth, job security is in their own hands, not the station owners. Fifth, no office politics involving people not connected to the show.
And, most of all, they run their own operation. They're the boss. Which gives them complete artistic freedom, with no format and no corporate suits or government regulators. They deal directly with the sponsors, distributors, etc. The buck stops far closer to them.
Quite a few of the bigger podcasters were people who got chewed up and spit out by the radio industry, such as Marc Maron, Adam Carolla and Phil Hendrie, to name only a few. Think they'd want to go back to radio? What's the point? I wouldn't be surprised to see Howard Stern eventually go the podcast route.