Primaries have to be harder because there is far less polling information compared to the general election. Yes, they were unable to predict the Trump victory, but if the get 90-100% of states correct in the general election, then I doubt that much has changed with how polling reflects the electorate.

I'm not a Trump supporter by any means, but 538 went way out of their way to ignore every poll that was put in front of them in favor of punditry.

The correct solution is just to move on. Too much of the media coverage of elections is about the horse race aspect, almost none of which is actually of positive benefit. Rather than spending so much time trying to guess winners, the media could spend more time and effort actually covering the actual issues of the election in a mature fashion, but it's been decades since they even tried.