jay z is a rap legend tryin to make it to a billi, married to beyonce. kanye a culture icon with reach broader than rap so i think they are probably special exceptions.
shouldnt forget jay z, kanye got young fans that grew up listening to them, and carried on listening to them as they put out successful album after successful album. alot of the artists that never made it to relevancy in their 40s just didnt have the discographies to keep up.
also i think rap is so widely consumed now that theres a big enough market for more than just the cool new rappers, its become less about what the labels are putting on the top shelves and on the radio, alot of different niches you can sell records in.