I mean, it's GPL. If something were to happen to him, anyone could just start their own fork. There would likely be some competition between forks for a while before two or three emerge as the successors eventually.
Linux is basically a national security issue at this point with how much runs on it. Linus is an interesting dictator because he's not bound to any corporation. I think some kind of kernel throne wars would be terrible for the community and could result in Red Hat or some other corporate overlord owning the kernel (not literally because of trademarks but effectively through the release and mainline).