In the real world there are powerful people, for sure - but the source of their power is their links with the society itself. "Power" in the real world comes from close engagement with society and developing links (or 'strings' that can be pulled) within it, not by separation from it to draw power from some "Arcane", "Martial" or "Primal" (or even "Divine") source. As a result, those with power may only be challenged through the source of their power, and they have built in defences (their networks and 'position') in that field. With "levelled" RPGs, however, PCs have access to an independent source of power, against which non-levelled characters have no effective protection. When power becomes "how many mini-games you have won" instead of "how strong a network of friends you have", the resulting 'society' is not what we are acquainted with on this world.
What you're saying makes sense, but I think the link is far weaker than you're presenting it: weak enough to permit levelling and the default fantasy setting without altogether much trouble.
Firstly, although at some level it's true that powerful people must work "within" the system, given how large that system has become, that's an almost meaningless proposition. Somebody with 1000 times the resources of another has that "money" by virtue of social convention, but it's very hard to take away - and having those resources means he'll win many supposedly fair fights (just look at how court cases go).
One could also make the argument that even in today's society, most nefarious powerful people get away with it - and that doesn't blow up society itself. If a company skimps on safety, they might hurt many,many innocents, but unless clear
personal blame can be shown, that just means some faceless company gets a bill, and the rich and powerful continue on their ways.
And, just like in D&D, there's almost always someone more powerful. Even the rich and powerful can't get away with everything, because others can still bring them down - particularly if they're united. Even the superrich may be jailed (even though they've got better chances) - and in D&D you might consider the deities and demigods or whatever other power - there's tons in lore - that may strike down others seemingly arbitrarily.
So long as your fantasy setting involves mostly harmless populace and just few rare powerful individuals, I think you can get away with levelling just fine - no it's not the most
likely set up, but you
can keep it reasonable and consistent. Those running large nations will need to benefit in some way from keeping the peace - but then that's easily imagined. And of course, D&D societies seem much smaller; which would be consistent with the idea that levelling isn't ideal for huge spread out societies.
And note that random acts of violence aren't really all that attractive to most. If there's any kind of society and ethics going on, you're risking at best vigilante justice - and even high levelled PC's aren't immune to "accidents". And you might disturb something or someone
with power - so why take the risk unless there's some gain? And given the way magic items work, there's no real gain unless you've got a massive network of support; high level magic is simply too hard to achieve. And if having a massive network helps; it'll need to be protected by those powerful, with whom you'd rather not mess - even if usually you'd get away with it.
So while levelling and rural medieval society are somewhat at odds, I don't think they're mutually exclusive - at least, certainly not so flagrantly that you can't design a framework in which apparent contradictions can be explained away. And since we're in a designed fantasy world, that suffices.