I could name you a lot of examples of series in which the first one was good, the second one was great, the third one was still good and everything after that a mere disaster...I don't believe that sequels can drag down original films. I also think with that attitude, we'd have never gotten Assassin of Kings or the Witcher to begin with.
If the next game or the game after or the game after that sucks, I won't buy it.
But until then, I will.
It seems that in almost every work of fiction there is a certain creative "climax" reached at some point, most often in the second or third work, especially in movies and games.
And I certainly don't want a game on Ciri, not after what they did to her in TW3...