The only thing poorly executed about Jon's fate in ADWD was that it was left as such an annoying cliffhanger. I don't think it was meant to be some totally-out-of-nowhere event, just one of those things that need to happen to get Jon off the Wall & to increase tensions/drama/danger at the Wall even further as we near the finale of the books.
And ADWD is not a "poorly executed" book, that's fucking ridiculous hyperbole. It has a lot of the best writing in the whole series. Maybe you could've shaved some pages out of Tyrion & Dany's chapters, but a lot of their chapters & most of the book otherwise is still masterfully crafted & extremely well written. Ultimately the book feels comparatively lacking because it just leaves most every separate bigger storyline without the kind of climax + resolution + a little bit of aftermath that AGOT, ACOK and ASOS had, but that doesn't mean the whole book was poorly written. They just feel bad for a GRRM/ASOIAF novel that so far have been pretty good at offering a little bit of conclusion at the end of each book, while also leaving plenty of room & opportunities for the future. The kind of cliffhangers ADWD has feel unsatisfying, but ultimately I don't think that negates all the good of the book. It just feels annoying NOW, when we don't have the continuation of the story in our hands.