Instead of having an active hand in the (partial) restoration of the world (remember that this "restored" world is but a few million spots of light still in a sea filled with Darkness) I tend more towards the notion that the seven PoH were "created"/chosen as a key part of the new "security system" that was supposed to keep the remnants of the light world safe as the border between the Light Realm and the Dark Realm (which I too believe already existed, but completely undetected and undisturbed) was broken through the apocalyptic fighting during the Keyblade War.
This new "security system" which involves the Door to Darkness, the Keyblade of Heart (which is confirmed to have come into existence only after
the Reorganisation of the world), apparently both Land of Departure and Radiant Garden (it was the opening of the door to the Heart of that world and its downfall which brought about the invasion of the Heartless into the whole Light Realm, furthermore the Final Keyhole is also located there) and the Final Keyhole, seems to be a sort of "replacement" for the old border between realms that is outfitted with several locks, with the Door to Darkness being the last one and the Keyblade of Heart (and thus the seven Hearts of the Princesses) being its key.
Land of Departure and Radiant Garden are possibly other locks which Xehanort systematically teared down during his "crusade" throughout the series. LoD already in BBS when he orchestrated the downfall of its Guardian Eraqus and then sunk the world itself in Darkness. Aqua transforming it into Castle Oblivion presumably protected the world's heart and prevented it from being completely destroyed, and thus possibly kept the lock intact, although weakened.
Radiant Garden Xehanort worked to bring down from within by assuming a place in the world's ruler's inner circle and just like Eraqus, Xehanort eventually orchestrated Ansem the Wise's downfall, followed by the fall of the world itself in which he more or less used the "help" of Maleficent again.
With both LoD and Radiant Garden either severely weakened or otherwise out of the picture, Xehanort went on to try and break the last one, which much of Ansem SoD's plan in KH 1 revolved around. Had Ansem succeeded and the door remained open, I guess this whole artificial border separating the realms would have collapsed for good and a flood of Heartless would have brought about the end, reaching Xehanort's goal of drowning the universe in Darkness without having to fight an apocalyptic war beforehand.
Back to the Princesses, they as a collective are said by Yen Sid to be the "replacement" for the true KH which was lost to the Darkness and they do NOT have the seven light shards of the X-blade which apparently several people falsely claim.
And the light from the broken Chi-blade was then divided into seven, to protect the number of pure hearts in the world.
This part clearly mentions the light shards independently from the "number of pure hearts in the world" which refers to the Princesses.
The "security system" apparently worked very well for a long time and the safety of the Princesses is imperative for it, which is why Eraqus stresses as much during BBS.
Now since the Princesses are the "replacement" of the true KH and also apparently a key for the border separating the light and dark realms, I come to the assumption that the original true KH served as much the same purpose for the original world.
It is said several times in the legend that the true KH was the source of all light (for the Light Realm), but since light casts a shadow, KH had one too which actually IS the Realm of Darkness. The true KH thus acted as a lock/security to its own shadow, separating the Realm of Light from the Realm of Darkness.
This would also explain why the X-blade is made of both Light AND Darkness. The X-blade is the counterpart, Guardian and
key to Kingdom Hearts all in one. Like the Keyblade of Heart is the key to the Door of Darkness, the last lock of the "new" security system in absence of KH (with the Princesses as substitutes, hence their importance to it), the X-blade is in actuality the Key of the original "security system" that separated the realms for good due to its own position, the true KH itself.
As the key for this lock though, the X-blade needs to be compatible with the true KH and therefore the two elements that make it up were taken from KH itself, it was made from both KH's light (the light half) and KH's shadow (the dark half).
It is said that the X-blade is the perfect crossing between Light and Dark, which is true insofar as it was created from the perfect Light (KH) and the perfect Darkness (KH's own shadow).
When the people became greedy for the light and tried to obtain the X-blade to unlock KH and thus gain its power, they forgot/ignored/didn't know that this would also mean unlocking the border between KH and its own shadow, causing the heart that is KH to fall to its own darkness, which is exactly what happened at the end of the war and what was replicated on a smaller scale in KH I itself.
The Keyblade War led to the X-blade being shattered, but by now I believe the X-blade shattered only after someone already successfully used it
. The true KH was unlocked, the border shattered and KH's own shadow (the Realm of Darkness) rushed forward to blank the universe in Darkness, swallowing KH itself in the process. With the X-blade destroyed, all means to relock the border were lost and thus no one could save the true KH nor the Realm of Light.
The hearts of the Princesses might probably have been the last hearts born from the true KH before it was swallowed and they were sheltered/saved by the children who eventually rebuild the world (much like Sora took in the "newborn" hearts of Roxas and Xion and Kairi with Naminé). As we know by now that hearts can spawn from other hearts, this might also explain how the hearts of the Princesses need not to be exactly the same hearts over all generations, as the hearts of the current Princesses might have been spawned by the hearts of a previous generation of Princesses and since we have yet to see a heart spawning a new heart of the opposite gender (Xion doesn't really count, as she wasn't spawned by a heart per se but her heart was built from a bunch of memories and then nurtured by connections), we might have an explanation as to why the Princesses are always all female: Because the originals were.
Those children who restored the world however eventually found a different way to reestablish the border that worked without the true KH and the X-blade, most likely using the hearts of the Princesses and the hearts of worlds like LoD and Radiant Garden, thus establishing the system we see in the current series.
Without the true KH though the Light Realm could not be fully restored, or as Yen Sid puts it: "even the brightest world will have its dark corners". It is entirely possible though that the surviving children did not even want
to fully restore everything as the memory of the catastrophe the citizens of the united world eventually spawned was still so fresh. This was why they established the new rules of all smaller worlds being isolated from each other, possibly limited the number of active Keyblades and repurposed them from weapons of conquest to weapons of protection.
Looking at KH I, it is Sora
who replicates the process from the end of the Keyblade War on a smaller scale. By using the Keyblade of Heart he unlocked his own heart which was swallowed by its own Darkness. In the process releasing Kairi's heart which also completed the Keyblade of Heart in the process.
With all seven hearts together (even if it was only for a short time), the replacement for the true KH was complete and the last lock (the Door to Darkness) was unlocked.
After the unlocking, the Keyblade of Heart shattered just like the X-blade and the border was about to come crashing down again. This time however the Princesses were already around (in contrast to the time of the Keyblade War) so they could hold the Darkness back, preventing the border from totally collapsing again.
Eventually, Sora and Mickey stabilized the border again by using yet another different way, by using a Light Keyblade and its fitting Dark Counterpart, which again
replicates the core function of the X-blade and
the Keyblade of Heart.