To put my thoughts into perspective on why I say Hydaelyn destroyed the world, I'm going to employ a metaphor.
The resting heart rate of a healthy human being is roughly in the range of 60-80 beats per minute. A trained athlete can have theirs dip down to around 40 beats per minute, and an unhealthy individual will typically have their heart rate spike over 100. While active, your heart rate will also spike to up to around 200 beats per minute depending on the activity. And you can't really control this. Your heart is one of the muscles in your body that works automatically, because if it didn't you'd die.
Imagine waking up one day and finding out that, if your heart rate ever goes over 70 beats per minute, you'll drop dead within seconds. That sounds ridiculous, right? Nobody could live like that. A more "fair" example might be breathing, which is reflexive and typically automatic, but can be done manually. Imagine having to breathe manually for the rest of your life, and only at a slow, steady rate. If you breathe too quickly, you die. Still pretty ridiculous, yeah?
In all of the Disciples of Magic questlines, the matter of using one's own aether reserves to cast spells is brought up. Conjurers have to draw power from the surrounding land. People with low aetheric reserves are kept from joining the Thaumaturge's Guild for their own safety. Black Mages who lack the training or outside means to control the massive amounts of aether needing for their spells can end up burning themselves alive from within. A Tonberry Scholar puts herself into a coma just trying to summon her fairy. Red Mages need specialized equipment to handle mixing white and black magics while avoid the ever present issue of death by spellcasting. All this is after six, then seven rejoinings, given that the story of FFXIV straddles between the five years before the seventh and the year after.
Now the thing is, the world wasn't always like that. There used to be a time when even children's capacity for aether greatly exceeded that of the most accomplished magic users in the present day. It's hard to believe that the spellcasters of that era had to worry about things like boiling their own aether or healing themselves to death. At least, not until Hydaelyn did her thing. And even if their greatest minds—what remained of them, at any rate—had the foresight to immediately warn everyone about what had happened, it's hard for me to believe the entire world made the transition smoothly. On the contrary, I expect that in the years following the event, all fourteen shards experienced a lot of death, the collapse and dissolution of civilizations and cultures, and so on.
Granted, the above is just speculation. For all we know, there were absolutely zero ramifications to Hydaelyn splitting the world aside from things like "people are no longer nigh-immortal" and "Amaurot somehow ended up under the ocean". I'd be pretty disappointed if that were the case. It's not as if any other "good guy" in the story is exempt from making terrible decisions—with lasting consequences—that seemed like a good idea at the time. Heck, Ryne came seconds away from causing a second Flood of Light and wiping out all life on the First and all she was trying to do was help us by being Shiva. Why should Hydaelyn be any different?
But even if there's some kind of substantial mitigating factor, like "Hydaelyn needed to do it or else Zodiark would enslave everyone" or "If Hydaelyn didn't do it, the evil space alien gorging itself on souls in the underworld couldn't be dealt with", or something else along those lines... even if something like that turned out to be in play, it wouldn't make the effects of the Sundering any less horrifying from my perspective. -5 doesn't become a positive value just because it's closer to 0 than -10, and bad things don't become good just because the alternative is worse. Which is why I think, should we ever get a full(er) account of what happened to the old world, the collateral damage of the Sundering should not be downplayed or glossed over.
... and that's it for this long-winded digression. Circling back to the topic at hand, I've always wondered how feasible it might be to establish gateways between the remaining shards as an alternative to further rejoinings. Instead of trying to force everything back into one world, networking what remains so people can move freely between the shards. Mind you, the last time anyone established a semi-permanent connection to one of the Reflections, it was Xande doing it with the Thirteenth/the Void, so it probably has a high chance of backfiring horribly.
Also, I apologize for causing anyone reading this to breathe manually. And again, my apologies.