"Almost always" is not always. There are more reasons for successful spawning than size, and humans can't duplicate that. Plus, it is well known that ancillary males successfully participate in spawning. And, of course, resident mykiss produce anadromous young and vice-versa.

Random spawning is certainly not the most natural way to go but any sort of artificial choice is similarly not natural.

The purpose of most hatcheries is to produce fish to harvest and they need to be bred selectively to optimize production in that environment. When you try to make hatchery fish "wild" or wild fish "hatchery" you lose productivity. The environments are different and require different adaptations.