Fallout has never been about realism. If it were, they still wouldn’t be able to rebuild cause everyone would still be dying of radiation poisoning and cancer. You shoot ghouls to the moon in a shitty prewar rocket in one side quest. Theres a whole dlc about getting abducted by aliens. I think it taking 200ish years for civilization to come back is the least of our concerns in terms of realism. If you can suspend your disbelief for everything else in Fallout I think people can suspend their disbelief for this.