The material for the show is almost entirely from books - although they have some free reign. They are pretty much leading right into The Blood of Elves which is where they go from multiple series of short stories into multiple novels.
Also if anything the only problem with the Ciri casting is that she's slightly too old. Does a fantastic job in the series, hoping she has great chemistry with Geralt and Yenn. She's really going to have to shine in season 2 which if they lead into Blood of Elves will require some significant work on her part as she becomes a "witcher girl."
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I'm not sure if they address it in the games - haven't played or seen much of the first two and they didn't really talk about it in the third - but there were multiple causes for the downfall of the Witcher orders. One of them being a pamphlet created by and disseminated by a wizard that decided it would be in the best interest of the world to get rid of them. It never says why but from the themes of the fictional universe it could have ranged from a petty rivalry with the wizards that first created the Witcher orders, being upset at the Witchers for not violating their orders of neutrality (they are supposed to stay out of politics and aren't assassins), a Witcher slept with them and left or slept with someone they love, or a Witcher killed one of their pet monsters.