Well I think Sapkowski is really within his rights to try and stay detached from the Games' storyline.
The games are responsible for much of the Witcher popularity. And since the developers continued the story right where Sapkowski left it, I would imagine that it would be really hard for him to tell a different story and discard the storylines of the games, since most people only know about that.
One has to take into consideration that Sapkowski had already developed ideas in his mind for future books after the Saga, but CDPR's games have made those ideas really hard to bring to life, since they would probably contradict each other.
I believe that's one of the reasons Sapkowski decided to do a prequel, to maybe take the time and see what happens with the games, and how he could write around them, because I can't imagine that him writing a book that discards everything that happens in the Games, would be a very popular decision.
Then again, he might have come to a conclusion that he can't marry both his "old" and "new" books and the games together. So he decided to write something "older".
I do hope he writes more books about the Witcher though, both in the past and the future. He is a really interesting and enjoyable writer. But I don't think one can find a way around the games, especially since there are different endings as well.
I bet I'd be frustrated if something blocked my creativity and ideas, even if it has made me popular. Frustrated, not necessarily angry at.