Intro
Clerics, Druids, and Wizards present so much individual power it's a wonder why they, eventually, stay around the prime material, or their plane of origin. (Archivists, Artificers, Psions, Wilders, and some other full casters are in this category too.)
What This Post Is Not
There comes a point where a character could become party-independent by covering the roles himself or/and with his minions. I can easily imagine such a scenario.
Specifically, What This Post Is
At what point is a character so self-sufficient that it no longer needs a society?
For example, it doesn't need to trade because it has or can readily produce anything it needs.
Level 9 Wizard Example
Consider a Wizard. At level 9, he can teleport. He can dominate Humanoids he likes. He can bug the gods with contact other plane. He can scry on anyone who hasn't blocked him. He can get Nightmares via lesser planar binding to cast astral projection, rendering him effectively immortal. He may have a loyal undead legion via animate dead. He probably has more spell preps than he'll use in a day.
The standard answers of why said Wizard hasn't taken over is because "There's a bigger fish" or "It isn't fun to play" or "It isn't party-friendly."
Well, yeah. By this point, the most logical reason someone is loyal to this plane is because it's rich in resources, or it's easily to conquer, or it's already yours.
If you go with the notion, "There's a bigger fish!" then that being would need to be more powerful. Logically, how would anyone rise to such a position of power if creatures are paying very careful attention to would-be rivals?
Regarding fun and party-friendliness, this could easily be an NPC. It probably already is.