That's a good way if putting it.
Storyteller was pretty much the headspace leader in the hobby in the 90s, as TSR was imploding, sure, and RQ was one of the first/strongest 'core systems' (BRP), and was essentially adapted into other games and genres the way d20 was 20 years later, just by a single company, rather than 3pps.
D&D has barely evolved, at all, and, while relatively few other systems go through multiple editions, those that do can evolve quite a bit - Hero went from an amateurish typewriter-font superhero game, to a Universal System the likes of GURPS, in 4 editions, for instance.
But, while so many other games just get published and never get a 2nd ed in which to incrementally change, in aggregate they've constituted a lot of evolution (with the kinds of indie games referenced by Campbell, above being 'more evolved' examples), which D&D has generally eschewed.
That's the downside of the network effect: to keep it going, the game can't afford to change much, even in a more flexible, 'evolved,' or more inclusive direction. So D&D, today, is not really all that different from the fad years, when we were brute-force adapting it to even the most wildly unsuited uses.
That's not just good for maintaining the legacy network and the decades of experience built into it, it's great for returning players, as well, still recognizably D&D, even if you've been away 20 or 30 years. But it does mean that the flexibility D&D does have (and that is confusing to Campbell, having experienced flexibility actually delivered by a system), is entirely delivered by the DMs constantly re-tooling it in spite of it's innate mechanical inflexibility.