in one way the job has not changed (he's running kernel development), but in many other ways the job has changed drastically every few years as the process has grown.
besides, if you like what you are doing, and are getting paid well, why should you change jobs?
a large part of the reason so many tech people in the US change jobs as frequently as they do is that HR departments don't allow very significant raises, so after a couple of years, the good people have learned so much more that they can move elsewhere and get a very large raise