Publishers just hype the next guy as the next big thing. Terry and the Pirates kept going when Milton Caniff launched Steve Canyon, and Spider-Man kept going when Steve Ditko/John Romita/Gil Kane/Ross Andru/Keith Pollard/John Romita Jr./Ron Frenz/Todd McFarlane/Erik Larsen/Mark Bagley/Steve Skroce/John Byrne/John Romita Jr./etc. left. 

When Ta-Nehisi Coates hits the end of his Black Panther contract, he'll either decide that was enough or he'll re-up, but either way, he and Marvel will have a perennial seller as a trade paperback. Everyone wins, and the Black Panther's profile is raised to the point that another famous author will want to write him or that he'll be a popular character again even without a superstar creative team. No downside there.