What does "D&D never dying" look like under this scenario?
Nothing particularly good for the game in the subsequent 10-20 years, give or take.
It's like asking what happens to the Roman Empire when the Empire part collapses. In brief: the social order shrinks drastically and won't meaningfully recover for "a long time" (but "entire civilization" is, I should hope obviously, something where "meaningfully recover" takes a hell of a lot longer).
Tabletop games in general will decline, not rise. D&D fans who already like whatever game(s) they have will just turn away from the hobby in general and isolate to their own group. Individual competitors might grow--I'd expect a lot more attention on Pathfinder 2e, for example--but the hobby as a whole would initially shrink, and that's gonna take out a lot of the smaller creators as well.
However, in the middle-term (roughly 3-6 years out from the initial collapse depending on its severity), it would almost certainly lead to one of the greatest flowerings of creativity in the D&D-alike gaming space we've ever seen, possibly even greater than the original wild-and-woolly days of 0e/1e. Because at that point,
there is no 900-lb gorilla anymore. That removes two of the hard, hard walls that shut out the creativity in this space: it removes the network-effect bias because
everyone is now in the "well what the hell do we play
now?" space, AND it removes the "no no no, D&D is
this one specific thing and NOTHING else, and if you change ANYTHING about what that one specific thing is,it's
RUINED FOREVER". (I'm sure the Transformers fans in the audience are groaning right now.)
When ALL of the competitors are necessarily small and building up from a small base,
and all of the traditionalists have had their world genuinely upended and
have to accept change or genuinely just...stop engaging with the wider hobby, that's a recipe for insane diversity and creativity. Now, in all likelihood, a lot of that creativity will be bad. That's just the nature of things. A lot of solutions weren't tried before because they didn't
work before, and aren't liable to start working now. But in that crucible of actual competition, where traditionalism and network effects aren't massively encouraging everyone to stay focused on the one biggest thing, I'm dead certain a slew of really interesting ideas will crop up. Some will be quite old ideas, and merely "new" to the D&D-alike audience who had been unknowingly ignoring them, but some will be new ideas too.
And then, in the long run? Something else will be what D&D was. Something else will rise to the top as "the" TTRPG game. D&D will be fondly remembered, the grandfather of the hobby. Assuming the "new D&D" (which obviously won't be
called "D&D") is primarily fantasy and non-modern, it's very likely the system would make conscious effort to reference back to D&D when possible. If it's more sci-fi or sci-fantasy, like
Shadowrun, then the direct D&D homages will likely be minimal, but even then I'd expect
some recognition, simply because of how important D&D had been and how long it held sway.
There's also the slim but possible chance that a different kind of status quo could develop. For example, instead of a single system, perhaps a consortium of TTRPG creators could come together and try to build a common platform (presumably both pen-and-paper and digital) so that many different RPGs can all converse and interact and even intermix, as much as their rules allow. I think that that possibility is extremely unlikely (it requires that
most of the creators care a great deal more about the philosophical positions and not very much about their pocketbooks, which is a delightful thing to hope for but not very likely in practice.)
TL;DR: An unfortunate collapse, a period of furious competition and creativity, and then a new status quo, probably similar to the old one but with a slim chance of something radically new.
