My dragons are relatively intelligent, but that doesn't mean they want to conquer the world. Intelligent =/= power hungry, and even the ones that are can only conquer so much.

The "Old guy with a ring of metal" has other reasons that he's in charge - let's face it, rulers aren't usually masters of physical prowess. But the world also isn't ruled by the level 20 fighter, a lich, the spellcaster who can literally cast wish....

The immortal fire breathing monster can be in one place at one time, and if it's only qualification is "Follow me or die" it's begging for some adventurer to come kill them- they're only immortal until someone puts a +2 sword through their chest.

The world takes more than "Might" to conquer and keep.