Of course, this is a bit of an oversimplification. In actuality, several developers have more-or-less supreme control over their particular subsections. There is a networking chief, a driver chief, and so forth. While Linus has ultimate authority, he seldom rejects a decision made by one of these sub-administrators.
So far, Linus has done an excellent job managing the development of Linux. However, even if he were to die, or quit, or take the Linux kernel in a direction that no one else wanted to go, he himself could not keep others from taking over the job, thanks to the GPL that Linux is controlled by. Even Linus can't prevent others from developing Linux.
Sometimes things don't make it into the kernel but are still useful to some group of users. For example, sometimes optimizing for one kind of hardware would break support for other hardware. Some users require paranoid security. Some developments are too experimental to inflict on everyone. In this case, people can (and do) provide their own patches to the kernel, available separately. These allow even greater customization without risking support for the vast majority.