I think this tension is longstanding within dnd. It is part a difference between sword&sorcery and high fantasy, but also pertains to the way that colonial scenarios are "justified" via an assumed moral lens, usually one connected to alignment. That is, in effect, the PCs are mercenaries for a colonial state, driven by greed, but their actions can also be glossed as defeating the forces of chaos and/or evil. The Keep on the Borderlands
. (Incidentally this dynamic--exploitative violence dressed up as justice or as a "civilizing mission," is central to colonial ideology).
The essentials kit is interesting in this regard. Phandelver has, for me, the feel of a settler mining town from a Western, with some fantasy gloss. There's reference to ancient history, but no sense that the human locals have any primary claim to the land or the mines. The antagonists are all alignment-evil, but they also have more prosaic motivations, namely they've all been displaced and are in turn displacing others. For the most part what they are after is food and shelter. The only antagonist acting outside of these types of concerns is an cult of an evil storm god (that could probably be reskinned as druids). Probably the way that most groups play this adventure is to defeat an antagonist in combat, level up, then defeat the next most powerful antagonist, justifying all of it via the evil alignment, but non-colonial solutions are on the table.
In truth, that S&S vs. HF dynamic goes back well before D&D and is rooted in the beginnings of modern fantasy, over a century ago. Some fantasy historians speak of two "streams:" One codified with Howard, who transmuted pulp adventure into fantasy, and the other with Tolkien (or, before him, Lord Dunsany and William Morris), who drew upon Medieval epics and mythology. Historian Jamie Williamson characterized these as "low" and "high" brow fantasy. It wasn't until around 1970 that fantasy became a distinct genre; before then, the low/pulp stuff was an offshoot of either science fiction or adventure pulp, and the high/epic stuff was often shelved with literature.
I mention this because D&D was born as the fantasy literary genre was being formed. The Ballantine Adult Fantasy series (1969-74) was an attempt to create a fantasy canon of the higher brow type. But more importantly, there wasn't really a "tension" between the two "brows," especially as they came together under one umbrella, that has continued to evolve and expand. As one fantasy literary agent explained to me, old forms aren't erased, they're just added to. Thus if you're an aspiring author and want to publish a more classic fantasy yarn, there's a place for that because it is an established part of the genre, even if the latest fad is more towards the gritty and subversive.
As far as D&D is concerned, the early years--the first decade or so--was far more towards the low-brow. But especially if we look at the fantasy roots, it wasn't as much a matter of colonization, but existing within a frontier land, or the remnants of a fallen, higher culture. Maybe I'm forgetting something, but I don't remember a strong theme of the PCs invading native humanoids and taking their land. It was all less distinct, and everyone in the same wild fantasyland. Meaning, everyone--humans, demi-humans, humanoids, and monsters--exist within a wild landscape, one on the fringes of civilization. The PCs aren't--and never were, really--the colonizers pillaging the natives of the wilderlands, they were explorers and even inhabitants of the wilderlands who co-existed with monsters and humanoids.
And it was always up to the DM what sort of backstory formed the context of the D&D world. Some DMs might, indeed, take a more colonial approach, that the PCs were like Old West frontiersmen pushing back the "Indians" (orcs). Others took almost the reverse, that humans and demi-human civilization was in decline or fallen, with humanoids and monsters invading from all around.
But my main point was that D&D can facilitate a wide variety of thematic qualities, and to centralize around a core theme at the expense of others is antithetical to the spirit of the game. IMO. Specific worlds and adventures can do that, but the core rules--the game itself--should be open-ended and flexible, and the community should be embracing, or at least tolerant, of diverse play styles. Thankfully WotC seems to get this, which is why they are bringing forth such a range of worlds. I think this "cosmopolitan" approach will be further crystalized once we see Planescape/Spelljammer and Dark Sun; the former because it literally opens up the game to "all worlds," and the latter because it brings back a more gritty, low fantasy world into the D&D family.
Anyway when it comes to settings, I do think they will try for a non-colonial, non-orientalizing kara tur or zakhara, if not as a setting than as an adventure, just because of some things some of the writers for VRGtR were hinting at on twitter. Whatever settings they do, they will have to be marketable and tightly themed for new audiences
Well, after the planes, Asian-themed D&D is one of the biggest thematic groupings that hasn't really been touched by 5E, so I could see that as well, whether it is east Asia, south Asia, or the Middle East. With D&D getting so big, I wonder if we see the emergence of some kind of "WotC International." A great place for that would be Istanbul, as it connects two continents and is close to a third, but I'm not sure if there's much of a Turkish D&D fan-base! But it may be that WotC will either contract out non-European fantasy worlds and/or publish them within an international context. I mean, how cool would be to see a Nihon Adventures
book written in Japan and translated into English?
The more interesting question for me is: when are they going to run out of nostalgia and classic settings?
Well, they're pushing out a bunch of classic settings, with four in 2021-23 (plus that fifth "cameo"). That still leaves several popular-enough classics they can draw from, maybe one every year or two from 2024.
But the brilliance of the 5E strategy, imo, is that it is focused on "stories and worlds," of which there are no end to the possibilities, and it also doesn't require a system reboot every decade or so, because the new base is more focused on those elements, not the crunch (afaik). There are still dozens of Magic planes they can tap into, and no end to new setting possibilities. Once they get through these next few classics, I imagine their focus will be on new worlds, Magic planes, and with some other classics sprinkled in.
I also don't think every classic will be a "one and done" product like we've seen with the previous setting books--all, really, except for the Forgotten Realms. Planescape quite literally opens up the game to countless possibilities, not just the plethora of material from 2E, but also new worlds. Dark Sun could also involve a series of products. In other words, I could see some of these "new classics" being new lines, with a series of campaign books coming out every year or two after the initial book.
So to respond to your question, they won't run out for years, even decades. And really, they probably aren't looking beyond the next five or so years, maybe ten lightly. By that point, the context will be different and the question may be irrelevant. But in the meantime, they have plenty of classic stuff to mine and publish for a decade or more.
As a simple exercise, here is an example of the type of classic worlds publishing schedule that I could see. This isn't a prediction of what I think we'll see, just an arrangement to see how it could look (core book in bold, campaign supplements in regular):
2022: Dragonlance, Planescape,
2023: Dark Sun
, Planescape campaign, Dragonlance part 2
2025: Forgotten Realms,
Dark Sun expansion (e.g. Silt Sea), Planescape expansion
Forgotten Realms expansion, Dark Sun expansion
2028: Gamma World,
Or something like that. that's usually 2-3 classic products a year, in addition to 1-2 new/Magic settings, 1 splat, and 2 general adventures, or 6-8 major products a year.
I think also that they'll know from sales which classic settings to expand on, and which to leave as one-and-done. Maybe Dragonlance takes off, or maybe Dark Sun is a dud. But they'll adjust accordingly.