But they did actually tell it is the last part of Geralt's story. April last year even, they explain how they may continue the saga, but with a completely new character:
"“I think good stories must have an ending," he said. "You cannot extend them indefinitely, and we felt that Geralt has had such an amazing adventure, and his saga is already so long and complex that this seemed like a good point to think about an appropriate finale for the story.""
"“The Witcher universe is a very big place with a lot of NPCs, a lot of characters, a lot of places we haven't shown yet," Szamałek explained. “So, we might return to it at some point…but we think that this is a good place to let Geralt enjoy his retirement and try something new.""
Personally, I even think if we could be seeing a completely CC for a The Witcher 4.
Things change. That was before release. Now they've had time to see how people felt about TW3, what people liked the most and what people want more of. Plus how much money it brought in and how many trophies sit on the shelf. Even now they've had to specify it's the last game with Geralt as the "main character."
I do think it is likely (and appropriate) for the torch to pass to someone else. Geralt has decided to settle down at the end of TW3. But the obvious candidate to takeover carrying the torch is Ciri. They already did a test run with her in TW3. People already like her; she fits the combat plus offers new options; is connected with the previous story and characters but has room to add her own new adventures and relationships. And Geralt, Triss, Yen, etc. would all be still important. 'The Witcher', the saga, is about these characters. If you want to bench Geralt as a protagonist, Ciri is really the only way to do it.
If you create a new character you throw everything you've created away. CDPR would be starting totally from scratch. A terrible business move. Plus you'd be trying to convince a lot of (now) unhappy fans to invest in this new character and give a shit. They'd have to waste a whole game just introducing them, new support cast, scenario, etc...
Ciri doesn't have this problem. She's already established - but her future is unwritten. She is the best of both worlds - the tailor-made successor to Geralt.