Well, I would say that it's both a crucial innovation that led to competitive advantage (path dependency), as well as a continuing advantage (Part 3 of the above).
It's not that other TTRPGs don't try to mimic the reward loop of D&D (and similar systems). It's that, for various reasons, other systems often aren't able to model the reward loop as well (for genre reasons, for rule reasons, for realism reasons, etc.).
I...personally think you're just wrong on this then.
Plenty of games assume a campaign, or at least something much longer than a one-shot. Even some legit actual
board games, like Kingdom Death, straight-up expect multiple sessions of play.
Call of Cthulhu,
Warhammer Fantasy Roleplaying, the various
World of Darkness games, various
Star Wars games, the nigh-innumerable systems Powered by the Apocalypse,
Shadowrun,
Cyberpunk,
Das Schwarze Auge, I'm sure I could list more if I went out and dug them up. And plenty of these, while either listening to, inspired by, or defying D&D convetion, definitely are
not D&D games.
Tons of systems, with different genres, implied settings, or perspectives pull off exactly the same reward loop as D&D. Your "(and similar systems)" sweeps under the rug easily
dozens of unrelated things. D&D retains its lofty position primarily through familiarity, marketing, and having been the top dog. Much like, for example,
EverQuest retained
its position as top dog for several years, before its aging mechanics and antiquated (often,
very specifically D&D-derived) player experience got trumped by the hot new thing,
World of Warcraft, which became enough of a juggernaut that it took some pretty serious controversy and missteps before it began to fumble--and it's still not clear that it's truly lost its way yet.
So it both had the early advantage, and it continues to have that system in place, that is harder to put into a "new" TTRPG that isn't "D&D-like." IMO.
But that's where I thought the discussion might go- how easy is it to re-create that reward system in a new, modern TTRPG that doesn't have the benefit of "grandfathering in" XP, leveling, loot, etc.
Okay so...how exactly can one even
do that?
You seem to be saying, essentially, "anything that uses these things is D&D-like," which makes the argument circular: nothing can use these structures without being D&D-like, and anything D&D-like doesn't count as a different system using these structures, no matter how unrelated it might be.
Like, if we applied this exact same logic to fantasy topics, you're basically saying that
absolutely everything which includes elves that are human-sized and at least
used to have an ancient and powerful society is 100% "Tolkien-like," and thus it's impossible to tell a fantasy story with elves in it that isn't Tolkien-like. Except...that we generally recognize that it's totally possible to have a high-fantasy story that
learns from Tolkien without merely
being Tolkien with a fresh coat of paint. Elves in Dragon Age, for example, are not (as OSP puts it) "gorgeous, elegant relics of a better time, ancient, wise, and more than a little alien." They're almost all either (a)
slaves or at least a racially-oppressed minority within human cities ("Alienage" elves) or (b) "savage"
wild folk who live in the forests and conduct guerilla campaigns against humans for current atrocities and past wickedness.
So: Is it even
possible for a game to include structures like experience, levels, etc. and not be, by whatever definition you're using, "D&D-like"? Because if not, then your argument is circular as I've said. You've defined the term so that it can't happen. If, on the other hand, there
is some way in which a game could use these things without being "D&D-like," then we can actually have a conversation about how such things could occur.