A couple of things. It is assumed that Geralt & Yennefer are dead, but we do not know that for sure as it was never made clear that they are dead. Also, Sapkowski (supposedly) is currently writing another book set in the witcher universe. But it is not clear on if it is about Geralt. We all know that Sapkowski doesn't play games & stated that the franchise that CDPR made is not cannon to the witcher world, so I don't know if the reason he is writing again is because of the book readers wanting to know the fate of Geralt, or is it because of the world wide popularity that was gained because of both the readers & the gamers. Maybe he is writing it because it will be a final act for the universe to be released after the 3rd expansion, so that way this game ends without anything else being made in game format. But that is just speculation on my part, but it make sense in a way.
BadWolf
Yeah sure there is some space for interpretation but as far as I know Sapkowski said that Geralts Story ends for him with 'Lady of the Lake' and that there will only be stories that play before that.
I'm not sure if he had never the intention to continue or just respects the games and CDPRs story.
For me it somehow looks that he liked the first game but as it got that much attention he somehow disliked the fact that "his" story got that famous because of the games. He was involved in the first game but not in the others. Also in the older interviews he was somehow more enthusiastic about the game. In interviews since TW2 he sounds more reserved like "they did a good job", just beeing polite and that he doesn't really care about the games.
...while whole Alvin plot was a bit against the lore, as it was established only women with Lara gene were refered as Elder blood, and were therefore Ladies of time and space. Males with the gene were usually either with suppressed powers or ended up being mad.
But Alvin ended getting mad, so this seems to fit.
I'm not saying story of Wicher 3 is completely perfect. I would rather see slightly toned down version, as the third Northern war was a bit too fatalistic, with almost all kingdoms destroyed/occupied (which has no background in book canon) etc..
How the war ended also bothered me pretty much. Especially that Temeria somehow disappeared although Anais was rescued in TW2.
But I won't say that there is no clue about a strong empire in the books. In 'Lady of the Lake' there are this small texts at the beginning of each chapter. Their content is dated somewhere in the future and almost every of them take place in the empire. Also the two ladies in the tower gathering information about Ciri and leading her way at the end also take place in the empire.