Remember, kiddos: we do not start with a conclusion and look for evidence to support it, we look at the evidence and draw conclusions from it. Things are false until they are proven true, not the other way around.

There is no evidence the Ascians only need the Eighth Rejoining / Umbral Calamity to come to pass for an instant victory. There is also no evidence Midgardsormr intends to lead the survivors of the Bad Future to settle a new planet. There is evidence the Eighth Rejoining will not give the Ascians an instant win, and there is no evidence Midgardsormr intends to lead the survivors thereof to settle a new planet. (Put in less flowery words he's basically saying he'll protect them as they rebuild.)

Things are false until they are proven true. That's science, kiddos!

(Why are / were the Ascians so desperate to trigger the Eighth Calamity? Well, they weren't desperate per sé... but the Source was still in good shape after the Seventh so there was no reason to delay, and they've wanted to off the PC since s/he became a thorn in their side during A Realm Reborn. Seems good enough reason to me.)