Other crystal spheres (if we use the concept from Spelljammer) or D&D worlds couldn't suffer that fate if gods don't allow it, not only gods about nature, but who protect their favorite race, they wouldn't allow a genocide. Even the infernal lords wouldn't allow their names fallen in the oblivion because they want those sinner souls for their own domains.
Githyankis were in DS and in the second try they noticed it was too hard to be conquered, but this doesn't mean the end of the visits. Maybe because it is too dangerous it could be like a trap against other faction, with a plan as "the treasure is in the other room". And I believe it is possible some natives from Athas could go before genocide wars. Or a secondary effect to the planar gate was being sent to a far future.
And if you want a future Oerth as DS only you have to recover the concept of time sphere from AD&D Chronomancer, like crystal sphere but set in an parallel world or uchrony. This would allow new options to create stories. Even this could be used as excuse for possible error of continuity in mass media adaptations.
Don't you miss the native genasies from DS what appeared in 4th Ed?
If the metaplot in DS continues and Borys is dead, and the rest of regions are "unlocked".... what will happen? I mean this could be like in a MMO where PCs after leveling up can visit harder zones... but D&D stories aren't "the monster of the next season will be stronger", this is not Dragon Ball. If some sorcerer-kings are killed, the new potential antagonists can't be only new faces with a higher challenger rating or power level.
How do you imagine the rest of the planet? And not only the dead land with its undeads.
* Ravenloft needs a good excuse to allow new "sandbox" domains. Maybe somebody want to use some ideas from other horror settings as "Obsidian Apocalypse" by LPJ Design, Shadow over Shatak by Fat Goblin Games or Kaidan by Rite Publishing. Somebody after killing hordes of zombi German shock troopers in Call of Duty will want to create a new domain as a secret laboratory with panzer-punk look. Other option is some worlds or at least some limited zones aren't within the demiplane of dread but they can be cursed or influenced indirectly by the dark powers, for example sending people as "trojan horses". Let's imagine a group of PCs escape and go to an unknown world ruled by a tyrant. They start a rebellion but this fails. At least the wear causes they regiment is weaker to resist a new attack for a second faction, with a pyrrhic victory, and then they are defeated by a new group of rebels whose origin is the land of the mists again.
* After reading an article about stagecraft technology used in the Mandalorian serie I guess making medieval fantasy movies will be cheaper than now.
