Will

Does he have a legal obligation to fans? No. Is there an implicit agreement between authors and fans that a started series should and will be finished? Yes, of course. We buy the initial installment(s) predicated on the understanding that the conclusion will be forthcoming. It's a social contract.

When high-profile writers like Martin or Rothfuss don't finish their works, despite years in which to complete them, it breaks that trust and drives down the likelihood that readers will want to embark on other multi-volume series. More critically, it makes publishers gunshy about taking risks by paying out advances for authors who are trying to sell multi-volume works.

The case might be a little different if Martin (or Rothfuss) just came out and said "I've moved on and don't plan to continue this series." But that hasn't been the case, which is why I give short shrift to opinions like the one expressed in this article. (Although I do agree there is no reason the vile behavior directed at him.)