Theros is being discussed elsewhere, but I'll say my opinion is that the MTG settings are guest settings. At best, it's an attempt to approximate playing in that setting without needing to make a whole new RPG to properly do it justice. Ravnica and Theros (and to an extent Exandria) are about as official D&D settings as Kalamar or Rokugan were.
A D&D setting was a setting designed for D&D first. It should reflect what is in D&D at the time it is being updated. There are a few reasons for that.
1. WotC wants to maximize profit. They do that by making their books as compatible as possible. They want you to buy Everything books and use them in Ravenloft, Spelljammer and Dragonlance. They want you to run Radiant Citadel adventures in Eberron or Greyhawk. It is not in their interest to sell you books that forbid or exclude other books from being used.
2. They want settings to have a light touch. Ravenloft got rid of Powers Checks and dozens of changes to spells and classes. Spelljammer got rid of the rules for losing spellcasting while jamming. Krynn's moons no longer affect spell power. A setting gets a few added options, but almost never anything that requires a second book modifying a different book. You can make a character using the only PHB and play them in any D&D world with only an additional option (like Dark Gifts or Bonus feats) added via the setting.
3. A small one, but settings with consistent options make organized play (Adventurer's League) easier to manage. Especially with players who drop in and out and aren't necessarily keyed into a settings particular nuance.
4. It creates a unified brand appearance. Certain design principles and assumptions remain constant across all settings. When the book says "D&D" you're getting a large collection of assumptions already baked into it. It's just up to the setting to flavor them (War. Space. Horror. Pulp/noir.)
Now you're probably going to say none of that matters to you, the DM of your own game. Those are concerns for WotC as brand manager and publisher. And you're right! They don't matter to your table. WotC doesn't care if you ban everything but humans and fighters at your table. But they care what is going to go in their books, and if you think there is any incentive for them to tell you to ban anything, you're insane.
The era of TSR, who made a bunch of completing RPGs that used some common resources but were otherwise unique games, is over. The trend has been towards the notion of unified game rules and options since 3e. There may be an exception on occasion (like kender replacing halfling) but the overall trend is settings that complement but don't compete with the core rules.
There are a lot of problems with this argument, I mean, it's really full of holes, but let me say, you may well be right about what they're doing, but it doesn't make it a smart move. One of the USPs of D&D as a brand, is that it has this large range of genuinely diverse/unique settings that people talk about. What you're saying they're doing, is, as they certainly have with Spelljammer, and arguably have with VRGtR (I wouldn't really agree with the argument), is putting out extremely shallow (to the point of missing basic setting information), bland, kitchen-sink versions of settings, which are all essentially the same setting.
It's just that does not seem like a long-term good idea given the D&D brand and its USPs. The response to this is usually an appeal to authority of the marketers and the magical "data" they have, but that's not a rational argument, and history shows it's obviously nonsensical - countless companies have done incredibly boneheaded things despite/because of "data".
Also, no I wasn't going to say "that doesn't matter to me", you're confusing me with someone else (I know who, but that's not me). I'm saying "This is a bad strategy that will damage the D&D brand longer-term".
As an example, if they bring out Dark Sun, and it's also ultra-bland, extremely short, and forced into being a kitchens sink (at least Spelljammer already was a kitchen sink), it won't be us old grogs who are mad (or we won't matter), it'll be a lot of younger players, because they are well aware of Dark Sun and the limitations and weirdnesses it had. That's part of why it's legendary. Part of why people who have never played it still talk about it.
Re: Theros/Ravnica, obviously the Kalamar/Rokugan comparison is beyond ludicrous. That's so laughable as to not be worth responding to. But in your haste to dismiss them, you missed what was actually going on, which is that they're essentially relics of an earlier marketing strategy, which was cross-selling MtG and D&D via setting books as a major goal. But what that also shows is all marketing approaches are transient. Right now we're in an era of incredibly low page-count, ultra-shallow, very bland/flavourless takes on various settings. Will that persist? I think it'll persist long enough to make sure Planescape 2023 is absolute junk, the odds are very good on that. It's a hard setting to do right, and easy to potentially make bland and dull (c.f Monte Cook's post-Faction War take, or 4E's take which derived from that), and I think they'll manage it (esp. based on the godawful Feats we saw).
Will it persist into 1D&D? I guess we shall see. There are merits to people having settings which conflict with each other, having distinct opinions and so on. Especially if you're moving people on to a subscription.
