Throughout the series, we have been told what would happen if mankind sundered the compact, mankind has no means to flee the destruction that will fall on them, Morriel and her waystone are gone, the town born are not going to be fleeing Athera in space ships. They do not have the knowledge to use one if they could get their hands on one still working. Even if they could unlock the south gate, those worlds are still not habitable, the west gate is also locked, North is dragons only and although we have yet to find out what the east gate is for, if people had been vanishing though it I would think we would have heard of it. It would not be accessible to mankind at the time they need a refuge. Sundering the compact is completely different to issues like black powder that would eventually lead to mass destruction that might endanger the compact. And even when the s’Brydion brothers ran afoul of the Fellowship with that, exile was not an option since South Gate was not working Sundering the compact is a lot different to doing something that might in the future threaten it. Either the Fellowship or the dragons will act to preserve the mysteries.
They fail to preserve the mysteries, Athera will also fail and end up lifeless, one way or another, there would be no peaceful solution to the problem. The Fellowship can restore the planet, they cannot restore the human life lost.
Arithon would have a plan, he is not going to risk mankinds destruction. Janny will likely keep us guessing till almost the end what is going to happen. But the townborn are in no way ready to use the mysteries to leave Athera, the Fellowship is not going to protect them from Lysaers vengeance or a dragons fury, so either the compact stays intact or perhaps they will be protected some other way till the situation is sorted out. But no one will be leaving Athera till it is sorted out.
The Biedar are free to leave when ever they want to, but are waiting for Arithon to do something, Arithon would need to take care of the Mistwraith and Lysaer before doing that it seems.
It is not clear how most of the dragons were convinced to leave Athera, probably yet another prophecy, but if the compact is sundered and Athera and the Paravians endangered, I would think the dragons would return to Athera. If they can be convinced to stay on the other side of North gate, we already have at least one live dragon on Athera, who will not be snoozing when it is time for her to act. There have been mentioned locations that might be useful to hide from a wrathful dragon, but they cannot hide forever.
I would be expecting the Fellowship to leave Athera at the end of the third age/start of the 4th age, probably the Biedar will as well. Mankind was still there during the 7th age. So they do survive the third age, Arithon also survives it. We find out in the Song of the Mysteries how they manage to do it.