If the proper writer could be found, he might actually improve the series. Martin has already "done a Robert Jordan" to his series, because the real death blow wasn't Jordan's death and Sanderson's subpar completion of the series, but the way Jordan overburdened the series by trying to pretend it was something it wasn't (consequential) and adding way too many subplots because he'd rather drag the series out and make more money. While Martin can't be said to be dragging the books out by making ten more books than originally planned (though I do remember it being 3, then 5 books planned), he has thrown in bad subplots, extraneous characters and needless exposition because he wants to make more money and make the series something it isn't (consequential). As storytellers, both Martin and Jordan were serviceable (at their best), but neither are good writers.

Still, as damookster has pointed out above, it would be hard to find a less satisfying and poorly paced ending than the one HBO served up in the form of Season 8 of GOT. While Daenerys was definitely going to end up evil - there is enough foreshadowing to hit the reader over the head multiple times - the abruptness of the turn was jarring. Furthermore, it is clear that Tyrion was supposed to be the ruler of the Seven Kingdoms at the end of the story. He is the Henry Tudor character. He is the only surviving Lannister and inherits the Lannister claims to the throne through that house. He is married to the head of House Stark (that marriage to Sansa was never annulled or ended), and thereby holds their legitimate claims to the throne. Finally, it is heavily intimated that perhaps his true father was the Mad King, which would make him the legitimate Targaryen heir if Daenerys is dead and Jon Snow, true to form, rejects his right to rule.