And we have evidence corrobating Venats take. The statements of the souls in the moon, Elidibus reinforcing how even after setting right the world the people “cried out in rage and despair,” “resuming our role as stewards do the star,” the Hydaelyn entry in the Unending Codex, the refusal of the Convocation to hear out the Hydaelyn factions concerns (5.2), and so on. The issues people bring up with the cutscene, meanwhile, are almost all associated with the timing and order of events, not the events themselves. It was shoved together condensed yes, but that doesn’t invalidate the fact that it tracks to what happened. I highly doubt the 1st, 2nd and 3rd beast all appeared at the exact same time and place, given what we saw in Thavnair and Garlemald. Does that mean I should ignore the Amaurot dungeon?
If the Ancients believed life a curse that they wished to be free of, and they knew that they would be reincarnated and live lives that would ultimately come to the same conclusion, then would they not logically follow the people of the Plenty and devise a means to permanently end their lives? And we have no idea what they intended for Ra La and the butterflies once their work is done. Once the dungeon is complete the entire world seems to become a barren wasteland with no signs of life. I think that suggests what their world would look like after everything.
This is the riddle that ultimately underlies why the Sundered were able to survive. You are right, being mortal doesn’t solve the problem of destroying your species, the risk is always there. But being immortal doesn’t just make it a possibility you will be a Dead End, it necessitates it. The notes left in the aplenty, the statements of the Ea in Ultima Thule, all suggest the only way for an immortal species to overcome the consequences of their immortality is to not be immortal anymore, to choose to relinquish that and instead accept mortality and weakness. It’s why the Plenty citizens wax fondly for the time when they were struggling. They understood that risk of failure success rings hollow. So yes humanity is still at risk, but they can still avert that end.
Emet Selch is the last person capable of keeping that secret. The man wears his emotions on his sleeve when it comes to his people, and to sit by while the man who doomed them all is across from them would be an even greater fear of emotional strength than his 12,000 year Ardor. The moment when he yells at Meteion about what right she has to take their lives and decide their worth still makes me shake my head as the absolute worst thing to say at that moment.