I refuse to believe that after the Witcher 2, CDPR forgot how to write compelling stories. I am positive, as you rightly pointed out, that the story is poor because of the decision to go open-world. The story in Witcher 2 (though flawed) was a tightly written narrative with enough complexity to be believable and with just the right pacing so that it always seemed like the story was pushing forward. The most accurate way I can describe the Witcher 3 story is that it is so game-y. It has reduced itself to the pathetic standards of the rest of the stories in the video game industry. The story exists mainly to show off gameplay features, and Witcher's most important feature is it's Open World. The story makes you ride across the Northern Kingdoms, all the while subtly asking you to be impressed at the open world (which is very impressive, lets be honest). It feels like a story written for a game, while the best part about Witcher 2 was that it refused to be bogged down by these low standards and crafted a story that was not tied to convention or a slave to its own medium. Witcher 2 joins Half Life 2, Braid, Psychonauts, Spec Ops the Line, Bioshock and Silent Hill 2 as a massive leap forward for narrative in videogames, while Witcher 3 is a step back to the standard of the industry.
I don't care about the politics being dumbed down or simplified because from the get go, it was clear the Witcher 3 was take a tonal shift away from political intrigue into more personal relationships, however the personal relationships did not pay off in any satisfying way. The writers made it clear that the Geralt-Ciri relationship was at the core of the story, but they never go anywhere interesting with it, even when they reunite, their relationship is a one-note song. I am particularly disappointed in the lack of Ciri-Yennefer moments, as unlike Geralt, who she viewed as the man she was destined for but not necessarily her father, Ciri explicitly saw Yennefer as her mother-figure. But we get no pathos from that, nor from any other relationships because the story is too tied in convention and busy making sure we appreciate the open world. It's the curse of all open world games that has yet to be solved. Great interesting side-quests, a relatively decent open world (though the foliage, tessellation and anti-aliasing problems make me think Witcher 3 is not nearly as future-proof as the gorgeous Witcher 2) but a story without any pacing or urgency, written for the service of the world.
As Geralt says: "Sometimes I felt like you were right in front of me. Other times I felt as if I was going in circles."
You know what Geralt? You're not the only one who felt that way!