Oh, there's definitely a gamut. Hopefully, my understanding of that basic fact is clear from the ways I've responded, in this thread. There's a minimum level of "real" support. The specific options I've given aren't the only possibilities -- just the ones I've thought of. As much as I'd love to see a full, 256+ page ECS 5E, I totally see why they wouldn't, especially if current printing costs would push that up to a $75 (or more) book, which sounds possible. Also, as much as I'd love one for Eberron, and would probably pay $75 for it, I don't actually need it. In fact, too much of a full ECS 5E would be just another opportunity for "drift", which seem to happen every time a setting gets reprinted. I've come to actually believe that the best option for setting support is a SCAG-length book that gives the setting-specific mechanics necessary to play in the world and enough of an overview to whet the appetite of the uninitiated to go to DM's Guild for third-party support or find the old books (including at the DM's Guild). I'm not a Realms fan, so I have no opinion on whether the SCAG served those basic functions for the Realms and people just expected something different or if the book didn't even do that.
I will say that one of the basic criteria for "support" -- and why the print product is so essential to it -- is that I want players new to D&D with 5E to have a clearly visible alternative to the Realms as a published setting. Some of this bias is, absolutely, because I loathe the Realms. I will totally own that. But, I also think it's bad for the game to be too tightly coupled to any one setting. The biggest benefit D&D provides, IMO, is the battery of ready-made stuff (monsters, spells, etc.) that can be quickly and easily recombined to make a new setting, tailored to a group's tastes. That can totally include grabbing a published setting and go.
The D&D system actually isn't that great. Class and level based characters just suck. You can somewhat redeem classes by turning them into a starting package of some sort (see Genesys), but the gigantic blocks of level-based rewards is horrible. It's only benefit is in the simplicity and ease of use. As long as you keep your customization inside the given box, all is good. This applies for GMing, too, with the selection of monsters and magic items available without work. If you don't fit in the box, things get pretty dicey. Long term, tying D&D to the Realms means it becomes just another setting-specific RPG, like Shadowrun, Vampire, Exalted, etc. Rarely does someone take the time to decouple systems from they're settings. So, when people tire of the Realms, they'll abandon the system.
I'm definitely not begging for the setting glut of the 1990s. We don't need 873 region and race books for each of three dozen settings. But, I think having 2-3, maybe 4, settings on the shelves would actually help the game, not hurt it, especially if the settings weren't just 3 different takes on vanilla fantasy. I don't think there's any good reason to publish Greyhawk, Dragonlance, Mystara, etc. because they really would just compete with the Realms. Eberron is probably about as close as you would want to get. Dark Sun is the other obvious legacy property that stands out as "different, but relatable". Planescape and/or Spelljammer could be sold as "optional expansions" to the Realms or their own stand-alone settings. Neither is my cup-o-tea, but I don't necessarily bear either any ill will, either.
One thought on "pure digital" did occur to me. Since DDB is the official online support for 5E rules, and they actually aren't offering (much) non-official material, a digital-only product that was sold on DDB in a way that was indistinguishable from a print product would probably work. Buying habits are changing and, someday, the web will replace the FLGS for discovering new official products. I don't think we're there, yet, which is why I think a print product on store shelves is a requirement for anything but "punt", but I could be convinced that DDB is approaching that tipping point -- or, at least is close enough for a compromise. Better yet, promote it there and co-launch it on DM's Guild, with print-on-demand.
Doing an Eberron book as a PDF downloadable from the WotC site would just come off too much like an over-sized UA offering, regardless of layout, images, etc. I don't even like launching it solely (or primarily) as DM's Guild because it's too easy to miss or lose stuff, there.
