"But I've read fanfics that are better than published books."
Me too! But I would argue that a fanfic loses a lot of its soul, its seasoning when it gets poached for mainstream publishing. It's like treating the live-action remake as 'legitimizing' an animated film when the reality is that it's a soulless cash-grab in a creatively bankrupt capitalist market.
I'm not saying a fanfic writer can't go on to enjoy success in mainstream publishing, in fact, I genuinely hope that fanfic writers can break into that market with their own original works (Hello, Homestuck DNA in the Locked Tomb series...), but I do worry about the precedent set by fanfics "breaking containment" and getting picked up by mainstream publishers.
The whole point of fanfic and its legal protections is that writers are writing it for love of the game--sure, some writers are working on building up an audience, building up their craft, and there's nothing wrong with fanfic communities being a sort of writing nursery where writers can build themselves up before jumping into original fiction, but it's also like... fanfic has a point on its own. It has a purpose as transformative fandom. It has a point in subverting and exploring the text and subtext of the media it portrays from various interpretations. If we start treating it as a rough draft for original fiction and not something in conversation with both a community and a preexisting media... something gets lost on all sides when the serial numbers get filed off.
I mean there's probably more nuance that will bleed into "Everyone can do what they want forever"-- I mean Japan has a lot of fanworks in its doujinishi market after all---but I dunno. Fanfiction communities time and again want to have their cake and eat it, too, and I worry that's going to continue biting us in the ass and keeping us on the margins.