Brother, it takes several hundred years to recover from a political collapse of a civilization. A few thousand is in the ballpark for a 90% reduction in population coincident with a similar loss in knowledge. A 99.99% reduction would be more like tens of thousands of years.

It's not just that we need to have people living on another planet, we need a fully self-sufficient civilization on another planet which is at least 200+ years of sustained effort probably more because it took that long under the relatively ideal and easy conditions of simply settling another continent. Then factor in that their civilization would be even more precarious than ours and face many more dangers.