“A ship full of scientists and supermodels” didn’t feel like a really rigorous examination of future history to me personally, so I feel like this may be less of a problem than you expect?
First, @doyce, what do you assume about the Nomad as a concept that makes this a problem? The Menagerie had an explicit alien presence (the Concordance, with multiple species of aliens showing up on Earth), and the Nomad just says “you were pulled away from Earth by something”. This could be aliens, it could be extradimensional beings, fairy creatures, whatever.
Selecting for the physically and cognitively fit, albeit by subjective standards, has plenty of SF precedent (certainly at the cinematic and comic book level). Off hand, Hugo Drax in Moonraker comes to mind.
The Concordance and a galactic order (and law enforcement). (As established in the Prime universe.)
The idea of lower level conflict and rogue armed religious cults that kidnap kids to use as child soldiers (2Kick’s Nomad story).
That a disaster could render Earth uninhabitable in such a way that the only/primary escape would be generation ship / colony ship, using only Earth’s resource (i.e., the galactic order lets us go hang, no intervention). (Mette’s Harbinger story)
I think 1 and 2 fit together without real conflict. We have that in the world today.
The real issue is squaring 1 and 3. Offhand ideas …
… whatever disaster overtakes the Earth, it’s a galaxy-wide thing (invasion of aliens, carbon-based life form plague, etc.). So galactic order has fallen apart. (That could be by lesser means, perhaps those bush wars becoming more serious.)
… in the interval between the present and the Great Disaster, the Concordance has had a serious falling out with Earth, which is basically left its own, under embargo of any help and the like. Where they think they are going when evacuating is unknown, but desperate times, desperate measures.
… meanwhile/instead, Earth has gone into an anti-metas phase, super-powers and hypergeniuses are either suppressed or treated as highly controlled government resources. Any nascent Terran space travel tech is lost or downplayed until it’s too small scale to evacuate the whole population, and bigger/slower travel is needed.
… actually, that raises the question of actually evacuating. Assuming Kirbycraft space travel doesn’t become a regular thing (any number of reasons), the only alternative might have been large-scale craft.
I haven’t actually raised this with Margie; to be honest, and bottom line, if the only objection is the Nomad part of this, I’m happy to change (she is just an NPC, after all, while Mette is a PC).
And, just to complicate/simplify things, Margie wasn’t hot on the “generation ship” thing, but “colony ship” – travel that might have been a decade or two, not ten or twenty. Which, to lift a population in the thousands or even tens of thousands, might have been the only way of doing it if the galactic order doesn’t intervene (“Prime Directive!”).