IMHO, I think what he's referring to is that those 2e settings began as supplements to the core game, but as the years went on, they veered down the rabbit hole, and each setting became almost a separate game by itself (q.v. Dragonlance's Fifth Age, RL's Domains of Dread). So a RL supplement had little use to a DS player, and vice-versa.
Yeah, the way forward isn't entirely clear.
Like, for me, I don't know why an RL supplement
should be useful to a DS player. If I'm playing a DS campaign this year and I am buying books on psionics and books on desert travel and books on mutant wildlife and books on druids and whatnot, a supplement on gothic castles and mad science isn't going to be high on my list o' things to buy (not that I couldn't use it in some creative way, just that it's not a big target). Likewise, if I'm playing in RL, desert travel and psionics is going to be less useful to me than a supplement about occultism or an adventure-book about a Victorian-era city.
It's pretty clear form history that WotC shouldn't be investing a lot of money in developing lines without much crossover - not every city-state on Athas needs a splat, not every domain in the Mists needs a sourcebook, there's rapidly diminishing returns for that. But if all I'm interested in is DS, you're just not going to be able to make an RL book that appeals to me, and trying to do that would just hurt the distinctive vibe of RL ("you can do sand & sorcery in this realm of gothic horror, too!" weakens what RL does best!).
The worst-case scenario is trying to shove every differently-shaped D&D peg through one round generic "fantasy adventure" hole. If playing Dark Sun and Ravenloft and Forgotten Realms all felt like the same kind of experience mechanically, there wouldn't be much value in the new coat of paint (orange or gray!).
I imagine the best case scenario is to use he settings to enhance the breadth of D&D - to take someone with one round hole, and give them
more holes. Even if I'm not playing Ravenloft, I might loot an adventure for a Halloween one-off, or grab a sourcebook for my Victorian-themed island in my steampunk setting, or grab Strahd and Castle Ravenloft and plunk it down in Greyhawk for a month's worth of play. Even if I'm not playing Dark Sun, I might run a John Carter-style campaign where psychic poison desert-lizards are on mars, and I could use a lot of Dark Sun monsters, or some psionics rules. And maybe someone will play Ravenloft one year and Dark Sun the next, and that'll be good, too.
The settings can be used as vehicles to shape your play experience, to change it up and make it different - not so much deep lore dives, as new rides at the park. This might be described as "different games," but using the park analogy, The Haunted Mansion and Space Mountain are different rides, different experiences, but they're all contained within the same park. They don't all appeal to everyone, and someone obsessed with one might not have any desire to go on the other.
....and now I want a D&D-themed theme park where I can go on a ride through the Tomb of Hororrs....but I'm weird.
Iosue said:
. 5e openly embraces all of D&D history, in spirit if not in specific product support, unlike previous editions. Chris Perkins has said on Twitter that the default setting of 5e is "the D&D multiverse."
Yeah, the risk there is that D&D becomes one thing - that in fighting against the settings being "different games" it all becomes the same game, the same thing, just slightly different. Which isn't that appealing to me. Yeah, you can do a dungeon crawl in Ravenloft, but that's not using what is interesting or unique about Ravenloft to its best effect.
I'm confident that 5e folks CAN avoid that fate, but I'm paranoid - I'm a big fan of different settings, and I hate to see their interesting edges filed off to fit more tightly in a narrow paradigm of what the "D&D multiverse" is.
...I mean, Eberron doesn't care about the Great Wheel, nor should it.