a basic law of MoR is that people gain maturity/competence in proportion to how much hell they've been through.

Why?

EDIT:

However, the actual literary logic is something more like, "Once you show Harry thinking his way out of Azkaban, it is no longer possible for him to lose an even battle to Draco - the reader won't believe it." I don't think the 'power up through trial' thing is actually unrealistic, I mean, if I come out of this planet alive I'm probably not going to be fazed by much after that. But it's the more fundamental literary reason why so many stories work that way. You will perceive that this also points in the direction of, "Being run over by a truck isn't the same as punching the truck to a standstill" in terms of whether you powered up after that.