clearstream
(He, Him)
You make a good case. Say we read my words as "they must first earn XP"; what I propose is to take the game mechanics sincerely as the world metaphysics. As you outline, it feels quite reasonable (to me at least) to suppose that there must be some other sources of it... and your intuition that those sources are a trickle compared with the less-than-zero-sum from adventure, chimes with my instincts.Must they? For the PC's, the easiest path to power is 'to adventure', to take on heroic quests and overcome challenges. But it's not at all clear that most of the XP that NPC have was earned in the same manner that PC's earned it.
For one thing, everyone is worth less XP than they have earned. Levelling through combat is a less than zero sum game that results in less and less available experience in the world. The large predators of the world might not be able to breed fast enough to create a reliable supply of XP for everyone that needs it, especially in a world where high level NPCs are common.
So I suggest that there is a secondary XP economy that PC's rarely engage in, but which is readily available to the wealthy and powerful. Even if this secondary economy only trickled in an XP or two per day, over the course of ten or forty years, it could result in a sizable accumulation of power. Throw in a few of the more typical hazards and dangers to overcome over the course of that life, and it wouldn't require a whole lot of adventuring to hit higher levels.
So what does that secondary economy look like? Well, it probably involves a lot of intense training, probably from a rather young age. It might involve the equivalent of bottled XP or rituals that transfer XP to the host. The various librams like the 'Manual of Stealthy Pilfering' represent one known sort of bottled XP, but it's possible that there are weaker less potent versions that still could be useful if applied with enough regularity. Earned XP could also involve a lot of mundane education, reading and study, especially of the ultimate class levels earned were decidedly bookish.
What I extrapolate from is that power, in our world, has largely been accrued through military force. It's not absolutely inevitable, but it is by far the norm. In our world, one person with a spear is not as strong as one hundred people with a spear, so a faculty for influence, organisation and logistics pays off for those who would rule. Economic power means being able to hire others to hold or wrest power for you, but that is because those others are otherwise divided: they are not a single puissant individual. In the darker past, personal interest in and ability to fight did allow a move to power.I also think it's not necessarily the case that political power gravitates toward the powerful. Political power right belongs to the powerful is itself a political and moral philosophy, and if that philosophy prevails everywhere it's just not very useful. I think a certain amount of durability is helpful just to avoid getting killed by every foe and rival that comes along, but that once you are above the average level by a bit, your leveled up enough. What really matters is a combination of right to rule, ability to influence and at least minimal competence so that you can do your job. And for a ruler, that might mean a lot more character resources spent on things that are the equivalent to 'Skill Focus' and less 'Weapon Mastery'.
For the purpose of the game all it means is that the NPC needs to be the sort of character that the PC's don't mind obeying, which turns out to depend on something intangible - personality and characterization. You can have a twelve year old boy on the throne whose just a 2nd level rogue, so long as the powerful in the realm such as the PC's don't mind seeing the boy stay on the throne, and are willing to work to keep him there.
In Faerun, one tier 4 person is stronger than a hundred people with spears, and yet they are scarce enough that there is a lot of space - a lot of resources - between them. That is why I think the task of organisations in Faerun is really to channel those super-humans into behaviour of the sort each evolved elite (i.e. the existing sets of tier 4 people) favours. The Red Wizards being a great example. They're often cast in terms of their opposition to external factors, but their most important job, which must occupy almost all of their time, is shaping and controlling their own membership.
Everywhere it is of vital consequence what high-tier persons do, how they behave, who they favour, what they choose to achieve. I posit this operates at an economic and political i.e. strategic level. Without ASIs, that boy sitting the throne is just not as cunning and persuasive as the bard or warlock. Others would sincerely prefer to work for the latter, if charisma and proficiency represents anything real in the world.