Yeah, there are other ways for characters to come of age, but not many that are fitting for the genre.
Yeah. It's not that I don't think it's possible to be an adult without your parents dying, but this is SPN, and they go for the archtypal stuff, not prosaic day-to-day change. Star Wars on backroads, you know?
I do agree that the theme of trust is not accidental, and that the only way for the Winchesters to survive is for them to relearn how to do that.