Obviously in a hatchery with controlled environments a very high percentage of fish survive early stages of life that would not otherwise make it. Think of things that can kill an egg - siltation, poor oxygenation, predation, etc...none of these factors help create more competitive individuals. Truly the problem with flooding a river with hatchery fish is decreasing genetic diversity. I am not totally convinced that hatchery fish are vastly weaker individuals either.
Keep in mind too that once a hatchery fish spawns in the wild, all forces of natural selection will affect that brood. I would suggest that because of that, naturally spawned fish that contain genes from a hatchery parent will still be competitive individuals too.
BC's philosophy behind its hatchery system is to augment wild runs. This is contrary to the US were hatcheries in general, are there to create fisheries.