It's been a truism for a long time that D&D undergoes fairly radical shifts every few levels. At least in large part due to the magic system, but, the level system does change the game for any class. At low level, say 1-3 for AD&D, 1-7 for 3e, Heroic Tier for 4e, your character really can't have any lasting impact on the setting. Not directly anyway. Sure, he might stop the rampaging dragon or end that slave ring, but, by and large, nothing your character can personally achieve at this level will blow up the setting.
At the other end of the scale though, your character does actually have the personal power to blow up the setting. The characters can directly challenge GODS. They could become gods. And, "having a lasting impact on the setting" is what Epic Fantasy is all about.
Think about LotR. Middle Earth is dramatically different by the end of the series. Sauron is gone, the elves leave, magic is going away. This is a setting that is very, very different than what it was at the start. Compare, say, Conan. By the end of the Conan stories, Conan goes off into the sunset and Hyboria is largely the same as it was before he came in. Nothing really changes. Life goes on.
This is one of the hallmark differences between S&S and Epic Fantasy.
So, I do disagree with the idea that this shouldn't be tied to level. Sure, your 5th level character could be king, but, Emperor? How long would he actually survive in that position? And, lets face it, every setting book out there pegs the movers and shakers of the setting at pretty high level. Could you imagine the head of The Harpers as a 3rd level bard? The Grand High Poobah of the Zhentarim as a 5th level wizard? A member of the Circle of Eight as a 6th level sorcerer?
The game already pushes high level play in this direction. The setting books follow the same pattern. So, why not have the mechanics to back it up?
To be fair though, I can see not doing this from a business standpoint. As I said, one of the hallmarks of Epic Fantasy is blowing up the setting. That makes for really, REALLY angry setting fans.

Not sure if putting this in the rules would be like having a loaded gun in the drawer.