So I'm actually going to revise my thoughts on this after re-reading
@Reynard 's OP again.
I never really fully addressed the main point he was trying to make around the near-death of TSR / D&D in the late 1990s -- what would alternate history look like, where D&D exists from 1974 to 1997, but dies off in its TSR days, landing in obscurity as a curiosity, a historical artifact of, "Hey, remember that cool thing kids used to do?"
Initially I said Star Wars would supplant it, but that was based on the erroneous theoretical assumption that D&D never existed.
If D&D existed but died in the hands of TSR in 1997, here's what I'd say would have happened instead.
In this alt history, the main hinge point is that Peter Adkison / WotC ultimately decide that D&D is a failed property and not worth purchasing. TSR ultimately cancels all product fulfillment orders, shutters its doors, files for bankruptcy. From this point it only exists in corporate name as a shell/front to pay off any remaining debts, a skeleton crew clearing house / payroll staff for the fiction publishing arm, and a contract-basis legal team to manage copyright protections. It basically becomes a functional equivalent of Harmony Gold in the Robotech/Battletech space --- it only exists to sue people who step on their copyrights.
In this alt history, Obsidian / Black Isle / Bioware never finish Baldur's Gate I, as the game is dead before they can even release it in 1998. Or if it is released, it's seen as a last gasp, the final great moment in D&D gaming history before the entire thing crashes down.
So --- what happens next?
The thing of it is, all of the principal "players" who eventually went on to create and influence D&D 3 and Paizo are now free agents.
Monte Cook. Skip Williams. Jonathan Tweet. Ryan Dancey. Vic Wertz and Lisa Stevens.
I think at some point, some conglomeration of this crew, plus Adkison, plus some other "old school" TSR folks band together and create what we now call Pathfinder, but do it ten years before it actually happened.
Dancey was already flirting with the basic concepts of the OGL in the late '90s. The general legal consensus is that you cannot copyright game mechanics, only the specific intellectual property bits that work within the mechanics.
Imagine, then, if the OGL system was created by some nascent melange of RPG talent culled from both dead TSR and broken off from WotC.
With solid legal footing to stand on, Nu-Paizo sidesteps any legal challenges from defunct TSR to publish its new, "open source" rule set, and releases it as "Pathfinder."
So instead of WotC owning TSR + D&D, Adkison instead jumps on the bandwagon with this plucky group of roleplaying enthusiasts who are certain they can redefine the fantasy RPG space --- with a fresh, revamped/revitalized set of rules that, while based on D&D, re-envisions the core system. It's both uniquely different enough (and better enough) than what came before, while familiar and playable enough to be popular with existing fans. It fits the MAYA (Most Advanced Yet Acceptable) principle of product development in the exact sweet spot to take off.
The Pathfinder core rules come out in 1999 or 2000. Then you get the Pathfinder adventure path publishing arm fired up. Within 3-6 months, the Golarion campaign setting and the Rise of the Runelords adventure path are on board.
The OGL publishing glut starts off much more slowly, as it's no longer ingrained with the World's Previously Most Popular Roleplaying Game, but within a year or two it starts to gain traction.
The adventure path subscription model slowly builds momentum. Golarion slowly but surely supplants FR as the "default" fantasy world. By 2005, the enterprise is picking up steam. There's still a few D&D 2 holdouts who haven't moved on, but word on the street is that Pathfinder is the true spiritual successor to the now eight-years-dead D&D.
The OGL publishing stream is now a fully-running faucet. Maybe Nu-Paizo strikes gold with some book publishing deals as well to get broader mainstream appeal, while the bread-and-butter core rules and adventure path subscription model continue to generate revenue.
By 2008 or 2009 --- Pathfinder 1e's actual, real-world release date --- Nu-Paizo is already the established fantasy RPG market leader.
At some point the CRPG side kicks in as well. Instead of "Neverwinter Nights" the CRPG, you have "Angels of Andoran" and "Demons of Cheliax" CRPGs.
With the OGL now well established, Adkison works with Nu-Paizo to develop a M:tG property. The Pathfinder adventure card game is still a "thing," but now is backed by the World's Most Popular Collectible Card Game -- branding, offshoots, spinoffs.
Somewhere in 2006 or 2007 Nu-Paizo acquires a Star Wars license, and something along the lines of Star Wars d20 / Star Wars Saga comes along and pushes the envelope farther.
When Gygax passes away in 2008, Nu-Paizo releases several homage pieces/adventures that hearken back to Keep on the Borderlands, or ToEE, while purposefully maintaining the needed copyright distance. Maybe a collector's edition version of Pathfinder with retro-art, etc. Think of it --- Pathfinder with Larry Elmore, Erol Otus, and Keith Parkinson art instead of Wayne Reynolds.
In 2009, Nu-Paizo doesn't even have to fend off a moribund D&D 4e, because it never existed. By 2010, Pathfinder is the new generonym for fantasy roleplaying.
This might even have an interesting side effect on White Wolf as well --- because suddenly, the early d20 craze isn't quite as frenzied. Maybe in this scenario, White Wolf doesn't try to jump on the new "d20 fad" quite so suddenly and dilute its creative braintrust. I'd bet that White Wolf actually ends up in a moderately stronger position long term, as they're allowed to focus on their own core brands and core systems, rather than having to chase the money from the d20 glut.
In 2009, Pathfinder 1.5 comes out --- which is nearly identical to what Pathfinder 1e actually was, a revamped OGL system that maintains backwards compatibility.
Spurred by another 7-8 year window of compatible content availability, Pathfinder continues to solidify its grip on the tabletop fantasy RPG market. When Critical Role starts streaming in 2015, they're playing Pathfinder.
In our new alternate history, by 2017 D&D is only referenced in historical gaming retrospectives --- "Where are they now?" or "Whatever happened to...?" Netflix specials.