The standard aftermath of a crawl: quarantine, then parasites from every corner descending to bleed the survivors dry.
Rishi lays out the normal ending for a world like Earth. If this were a normal crawl, the system would go into Quarantine for some time afterward; it would eventually open up, and parasites from all corners of the galaxy would descend and bleed the survivors dry. That, he argues, is why the millions of humans still struggling on the surface will need friends — and why their children will need the naga.
He frames both branches for Carl: if it ends the usual way, the other planets in the system will be settled whether humanity likes it or not, so the Naga Treaty Offer costs Earth nothing it was going to keep. If it does not end the usual way — if the Macro AI keeps spreading and the collection never happens — the treaty is where the naga gamble truly pays off.
The default grim aftermath the naga treaty is designed to hedge against.