One of the things I find interesting about fanfic in this context is that, unlike those other paths, many (I would say most, possibly even nearly all) of the people writing fanfic are not doing so to learn how to write. They care about improving their skills only to the extent that it helps them tell better stories, but they're doing it as a hobby. They're not planning or intending on making it a profession. It's filling in the corners of lives devoted to something else.
My personal feeling, heavily influenced by free software I'll admit, is that this makes it a better teaching method, if a more scattershot one, since the people who really learn from this and who go into writing despite not having intended to do so would be doing so out of love. They would have found they loved writing by writing, doing it strictly because they wanted to. I think the most effective early learning is often the kind that sneaks up on you and ambushes you when you're just playing.
Of course, if they want to go on to be professional, some amount of follow-up structured learning is probably also required, even if that structure is just thinking hard about what you write and reading lots of what other people write and working out your own toolkit. But I think that can happen later.
My personal feeling, heavily influenced by free software I'll admit, is that this makes it a better teaching method, if a more scattershot one, since the people who really learn from this and who go into writing despite not having intended to do so would be doing so out of love. They would have found they loved writing by writing, doing it strictly because they wanted to. I think the most effective early learning is often the kind that sneaks up on you and ambushes you when you're just playing.
Of course, if they want to go on to be professional, some amount of follow-up structured learning is probably also required, even if that structure is just thinking hard about what you write and reading lots of what other people write and working out your own toolkit. But I think that can happen later.