I don't think it's lessened the threat. As far as resurrected characters, we have:
Gregor Clegane (likely)
Jon Snow (likely)
Even though the books have four (likely) resurrected characters, at no point am I willing to believe that Grand Maester Pycelle is still alive, or even Kevan Lannister. They're dead because there's no realistic way for them to return.
Gregor Clegane has mad-scientist Qyburn at his side. Makes sense.
Jon Snow can be saved by either the White Walkers, Melisandre, the Old Gods, some other strange force. Makes sense.
If Jaime Lannister were to be cleaved in half in Book 6 and left for dead, I don't think there's a good chance he'd be back, unless Qyburn was with him or he has an old priest and a Red priest.
Still rooting for the Hound, though.
The Riverlands would be leagues better than the Dorne storyline. The time spent sending Jaime blundering around the countryside could easily be spent with Jaime on a mission to the Riverlands. Keeps the Freys in. Keeps the Tullys in. And you can still get Dorne in, and instead of using Jaime and Bronn to link the Sandsnakes to the main story, you use Ellaria Sand. The watchers don't need Jaime there to understand Myrcella/Trystane/Dorne.
I don't think the Blackfish is a big enough character to take her place. The greatness of Lady Stoneheart comes from her role as a great Tully and Stark. Blackfish was a solid character in the book, laughable in the show. Watchers enjoyed Catelyn, as downplayed as she was in the show.
Brienne namecheked Catelyn a whole lot last season, and that led to a great deal of disappointment.
I think Sansa has other plans in mind in the books, namely coordinating the Vale against the Boltons and entering the endgame with armies and a marriage under her belt.