Theymos was just one of the global moderators then he became one of the admins then all the other admins left and he is the only one who remained so he became the head of Bitcointalk, it's by fate that he become the head of Bitcointalk and so far Bitcointalk is doing great supporting Bitcointalk advancement and enlightenment.
That's always the nature of having a community forum, next time there will be new promoted admin besides cyrus from being a moderator. But since theymos is just like in his 30s i guess we will still see him as the head for several years unless he resign early and pass his reign.