Idk, they can generally replace a writer, even the main one, if the ratings are good enough. If it was on the fence, he might have pushed it over into cancellation territory, but no one who watches the show equates his name with it really. It's not like Spacey and House of Cards, or a producer who owns the rights to a show. In those situations, you basically have to cancel it or suffer the consequences. In this case, you swap a writer and keep going.
Maybe the next season would have felt 'off' with a different writer, and then it would have gotten cancelled anyway, but they'd normally try at least. But, it's not like he wrote every single episode and every line of dialogue anyways. Plus, it's being adapted from novels, the core concept already existed.