I was fairly disappointed as well.
There's already a dozen worlds D&D isn't supporting, and they choose to bring a new world into the fold. At the expense of classic settings.
I've played two hands of Magic in my life and never bought more than a starter deck, which I gave away to a friend. And I've never seriously been tempted to get back in. I have no affection for its settings. But I don't really hate the game. So, that it's a MtG world doesn't phase me much (I can totally understand the reasoning behind that) but I think I would have reacted just as negatively to a brand new world. Because it means far lesser support for the settings we DO care about.
Yes, it's something cool and new. Something different that we really haven't seen before in D&D... but ANY 3rd Party company can do something new. Morrus started a
list of campaign settings, and there's twenty-five "new" settings you can play in. And that list is *only* for 5th Edition settings and doesn't include edition neutral settings.
New campaign settings are a dime a dozen.
But ONLY WotC can do Dark Sun or Planescape.
*
While I don't agree with the rest of the OP's first post, this does echo my thoughts.
Ravnica does bear a lot of similarity to something that longtime D&D fans would love: Planescape and the city of Sigil. It isn't, however, Sigil. In fact, its existence forces one to ask, "where is the design space for Sigil now that Ravnica is being released?" It seems to me that the existence of Ravnica makes an official Planescape product less likely, as so many of the concepts that would have been covered in a Planescape product are now being covered by Ravnica. It's like WotC spent months building up the idea of Curse of Strahd and instead gave us Curse of Innistrad. It would probably still be a cool product, but it would be a huge let down compared to Curse of Strahd.
And as the OP says, now there's far less chance of a Planescape product or product focused on Sigil. Just like
Tyranny of Dragons pretty much killed any chance of a Dragonlance adventure, or
Waterdeep leaves little room for a Castle Greyhawk style adventure.
Similarly, WotC is being
very conservative with its releases for 5th Edition. And trying hard not to compete with past products, which stay on the shelf. So releasing this product means we're unlikely to see a hard copy of another setting anytime soon, to give Ravnica enough time to move as many copies as possible.
And it seems unlikely that they'd release a lower cost product like this (where they can use assets from the MtG team, such as art and the setting bible) if they expected another setting to do very well; they would have saved this for the follow-up.
So this makes it far, far less likely we'll see any other campaign settings in stores. Other settings will make do for the "Eberron" treatment, where they get a PDF update.
But, in fairness... the weight listed for Ravnica on Amazon is pretty small. Comparable to each of the
Tyranny of Dragons books. So it could be 96-pages. Which would mean the
Wayfinder's Guide to Eberron is significantly larger. Still, it's unlikely to make any new Eberron fans. The DMsGuild is pretty niche and most players don't go there. I wouldn't be surprised is a large number of players hadn't heard about the product.