Em is a superstar, but his influence only carries so far. Why Cashis is not on the radio is proof of that.
We'll put it this way.
Troph 1 - Record company
(Is vital to Eminem or anyone selling extremely well. He has a large fan base but most of his customers are casual listeners)
Troph 2 - Eminem
(Yep, he's a super star)
Troph 3 - Artists directly under him
(D12 for example)
Troph 4 - Artists under D12
(Lesser known artists - Fuzz Scoota, and the other people on D12's mixtape)