If you get to a sufficiently-high level of "realism", D&D game mechanics and fuedalism can't coexist anyhow. All of human history prior to around the 17th century is based on the fact that a "military man" is within a certain range of power. You can have better or worse, like how the Roman empire had a more reliable way of getting more-per-soldier than the competition of the time, but in general throughout history, there's definitely a range on human power. Everything is so deeply based on that model that we can't even see it. Even as I describe this you may be going "yeah, but what about...", but bear with me for a second.
In these style of games, though, there's generally exponential power growth. One level 20 warrior can take on an absurd number of level 1 warriors. With modern games being so much larger and more complicated it's not impossible to find builds where a level 20 (or 40, or 60, or 100...) warrior can defeat arbitrary numbers of level 1 warriors. Moreover, the leveling mechanics are such that the things you do to attain levels are only loosely correlated to the skills you obtain from those levels, e.g., why would killing a bunch of kobolds suddenly allow you to cast two fireballs instead of one?
This breaks fuedalism in ways both subtle and gross. If the King is level 20 (or whatever), he has little to no utility for your Level 3 warrior's oath of fealty and the several dozen Level 1 warriors following him. In the real world every oath of fealty is some incremental boost in power and you may need everything you can get, but this oath of fealty is just a waste of your military's food.
So what would it look like? Well, you may note I time-bound my claim above that soldiers were somewhat range-bound in capability. Clearly modern militaries are wildly disproportional in effectiveness per soldier. It's been that way ever since the gun became a practical military weapon and has generally gotten worse over time. And what do we see today? Broadly speaking, the people with militaries have power and offer nothing like feudalistic loyalty in return. Loyalty is a one-way street where the plebs are beholden to the militaries, but the only loyalty the militaries have back to them is mostly based around the fact the plebs are still the supply line, so you can't actually kill them all, but you sure can kill a lot of them if you need to in order to maintain power. If you feel this is an inaccurate summary of the modern West, look beyond the modern West; there's a lot more to history than just the modern West in the past ~300-400 years. And it is, of course, a single paragraph merely sketching a hint of a broad shape, not a PhD thesis; I'm well aware that this is a very fuzzy picture. But the point I'm trying to make is not a positive one about the details of the sketch I'm making here; it suffices simply to point out that A: we actually have much less balanced "power per person" in the real world (though not driven by "leveling mechanics") right now and B: the resulting social structures that have been semi-stable now for centuries look nothing like feudalism at all.
>Moreover, the leveling mechanics are such that the things you do to attain levels are only loosely correlated to the skills you obtain from those levels, e.g., why would killing a bunch of kobolds suddenly allow you to cast two fireballs instead of one?
Interestingly enough, this is subverted in the first edition of AD&D- XP for killing was negligible, you got most of your XP based on gold/treasures amassed, once you hit the XP requirement for the next level you had to pay a mentor or trainer NPC ridiculous amounts of gold to have them train you for in-game weeks to actually gain it. If the DM felt that your characters play style did not reflect your classes archetype in a way that would allow them to "learn" ie if you played a magic user and never cast spells or used magic items he could deny you the level outright or impose a leveling or XP penalty on you. But understandably enough getting a report card from your DM every session on how you're playing your character "wrong" wasn't popular enough to stick around as a mechanic.
See also the common sci-fi story such as Skylark of Space or Microcosmic God or The Stars my Destination. Those are oldies but goodies, in all of them the hero experiences major leveling in capability by pulling up on their bootstraps.
Newer literature is more sophisticated as the hero becomes more socially involved. It definitely happens in Pohl’s Heechee series and also Vinge’s The Peace War but you don’t see it in The Terraformers by Newitz as the characters are born into the post-human.
Commercially there is a lot to gain from ‘future histories’ but they have a way of blowing up: exponential leveling can be sometimes contained, as in Niven’s Known Space but such a system can be killed by any bad idea, and that one got two.
In these style of games, though, there's generally exponential power growth. One level 20 warrior can take on an absurd number of level 1 warriors. With modern games being so much larger and more complicated it's not impossible to find builds where a level 20 (or 40, or 60, or 100...) warrior can defeat arbitrary numbers of level 1 warriors. Moreover, the leveling mechanics are such that the things you do to attain levels are only loosely correlated to the skills you obtain from those levels, e.g., why would killing a bunch of kobolds suddenly allow you to cast two fireballs instead of one?
This breaks fuedalism in ways both subtle and gross. If the King is level 20 (or whatever), he has little to no utility for your Level 3 warrior's oath of fealty and the several dozen Level 1 warriors following him. In the real world every oath of fealty is some incremental boost in power and you may need everything you can get, but this oath of fealty is just a waste of your military's food.
So what would it look like? Well, you may note I time-bound my claim above that soldiers were somewhat range-bound in capability. Clearly modern militaries are wildly disproportional in effectiveness per soldier. It's been that way ever since the gun became a practical military weapon and has generally gotten worse over time. And what do we see today? Broadly speaking, the people with militaries have power and offer nothing like feudalistic loyalty in return. Loyalty is a one-way street where the plebs are beholden to the militaries, but the only loyalty the militaries have back to them is mostly based around the fact the plebs are still the supply line, so you can't actually kill them all, but you sure can kill a lot of them if you need to in order to maintain power. If you feel this is an inaccurate summary of the modern West, look beyond the modern West; there's a lot more to history than just the modern West in the past ~300-400 years. And it is, of course, a single paragraph merely sketching a hint of a broad shape, not a PhD thesis; I'm well aware that this is a very fuzzy picture. But the point I'm trying to make is not a positive one about the details of the sketch I'm making here; it suffices simply to point out that A: we actually have much less balanced "power per person" in the real world (though not driven by "leveling mechanics") right now and B: the resulting social structures that have been semi-stable now for centuries look nothing like feudalism at all.