I haven't read the whole thread, but I think there's a common misunderstanding among long-term D&D players: that the new and very large D&D player base is as interested in endless splats and options, or rules minutiae, as many of them are.
I just don't think that's the case, but rather that the majority of the tens of millions of D&D players are far more casual in terms of how they play the game. The focus is on the next story, not endless crunchy options. In fact, I think crunch plays a third fiddle to adventures and worlds, which is reflected in the fact that of the thirteen hardcovers published from 2018 on--including Candlekeep Mysteries--only one of them (Tasha's) has been a player's option splat. The other twelve include one monster book, six adventure books, four setting books, and one campaign/rules hybrid (Acq Inc). Optional rules have been sprinkled throughout, but the point is that the focus is on worlds and stories (10 of the 13 being of that ilk).
This means that the edition cycle is not tied to running out of new options to print, in two ways: One, people don't care as much, as long as there are new worlds and stories to explore; and two, options have been more spread out, and generally lighter. DM's Guild fills the gaps for any kind of niche.
Couple that with the whole notion of an evergreen edition, and I think a true "6th edition" is not forthcoming anytime soon, if ever. I don't know how WotC plans their publications, but I imagine they've got a solid plan for the next few years, with strong notions of where they're going for the few years beyond that. I'd be surprised if "6E" is in either range.
So my guess is that the next few years, say 2021-23, will be more of the same: fleshing out the worlds and stories of D&D, throwing in some new options. We'll see expansions into the planes and other worlds, and maybe a surprise or two along the way.
2024 will be an important "taking stock" year. It is not only the 50th anniversary of D&D but the 10th anniversary of 5E. My prediction hasn't changed: I think we'll see revised core rule books that take into account the adjustments and expansions of the previous decade, but not an explicitly new edition. I previously predicted something akin to a "5.2," but I think 5.3 to 5.4 is more likely now, although still fully compatible with 5E. Meaning, whatever revisions are made, they'll stop short of "5.5." The revised core rulebooks will be published with the idea that you don't need them to play 5E products, old or new, but you're going to want them.
When might a truly new edition of 6E be published? Well that depends upon factors that are hard to predict from where we stand. When will D&D peak? And after it peaks, will it maintain most of its popularity in a long plateau or will it decline quickly? What about trends within TTRPGs - will some new idea or trend emerge that will force WotC to evolve (in a similar way that the indie revolution of the 90s led to the consolidated d20 system of 3E)? Will technological and entertainment factors (e.g. VR) play a part, and to what degree and when? And of course larger socio-cultural, health, economic, and environmental concerns; we live in troubled times.
But assuming some degree of stability, and a reasonable view on technological progression, I would guess that we won't see a new edition for a decade or more. We'll need to see D&D peak (maybe sometime in the 2021-24ish range?), plateau, and then start showing signs of decline (2026 or later, if at all?). And then it will still be a few years before we see that new edition, so I'm guessing 2030 at the earliest.
But again, not only is any such prediction impossible to make considering the current context (both of D&D's popularity and the complexity of factors involved, both within the industry and the world at large), I'm not sure WotC will ever explicitly speak of a "new edition." Or rather, "edition" will mean something different than it has for the last 45 years: not as much a new version of the game, but an evolution of the D&D game presented in a new way. Maybe that is somewhat semantic, but I'm thinking more along the lines of the shift from AD&D 1st to 2nd edition, than 3E to 4E or 4E to 5E. For that to occur again, I think we'd need to see a complete collapse of D&D's popularity, followed by a hibernation period as the property was sold and re-envisioned by a new company. But I just don't see that happening, at least from where we are today.