You are right that I wouldn't call Witcher a role playing game. It's an action Adventure game. You do not choose or create your character. You are given one.
I would still call it a roleplaying game. I think C&C games are an extension of the concept of roleplaying, plus the Witcher games have had many social elements such as hairstyles, and the dating sim mechanics that were definitely meant to be a bit parasocial.
I think character creation is definitely an RPG mechanic, but not a necessary one. Sometimes you're just given a character to roleplay as and I think that's fine.
I don't think whether a silent protagonist fits or not has much to do with the genre. If the game is in first person or the NPCs are not voiced either, silent protagonist works fine.
I don't think that's quite it.
I think that if you've already read the entire item of dialogue while selecting it, then by the time you make the selection, you've already kind of seen the line play out in your head. Having it spoken aloud again is a bit redundant at that point.
However, some lines of dialogue don't really benefit from a choice. For example, when you meet Astarion, he tricks you into looking for a "brain thing". After agreeing to look around, your character spends several seconds gazing around at the bushes without speaking a word before seeing a wild boar and expressing silent shock. There are other points in the conversation where Astarion says something, apparently expecting a response. The camera cuts to your blank expression for a moment, and then it cuts back to Astarion and he just continues talking. These are awkward silences that in any other story would be filled in with some kind of chatter.
Maybe another way to help with this would be to pop up a dialogue prompt with only one option so that the player character is still speaking? I felt this worked pretty well in Disco Elysium. (Still ought to be a bit of grunting when he wrestles you to the ground though.)
Still, I don't think there's any real downside to just having the game fully voiced. I quite liked the fast pace of the way dialogue choices were handled in Mass Effect & The Witcher 3 where you would preselect the dialogue based on the "idea" of what you want to say before seeing it play out in detail.
