Personally I like the structure imposed by playing a protagonist with a set and interconnected history with the gameworld. Although I certainly enjoy creating my own chars in games, I feel games that allow this lack the depth of storytelling and immersion possible when the char is "built-in" rather than "dropped-in" as your idea suggests. For me, when I play a Zelda game, I want to play Link. Now that I've enjoyed Witcher 1-3, when I play a Witcher game I want to play Geralt. When I play Max Payne... well you get the idea.
By having an imposed structure on a character and which you then assume that character's role, you have an RPG in which you can literally become a part of the world with your character, since the character whose identity you are assuming was already a natural part of that world. Whereas being able to create your own character simply lets you "drop-in" to the world which doesn't allow for your backstory, character theme, personality, etc to be truly integrated with that fantasy universe. It's not as if a compelling and interesting backstory you develop and carry around in your imagination can, in any way, be truly integrated into the game world in which the other characters are aware. In-game characters can't react to this backstory as programmers clearly can't anticipate any and all personas players could create and imagine.
Having a pre-set backstory for "Geralt of Rivia" allows for programming bounds that allow for these interesting quests, dialogue interactions, complex emotional elements between characters, meeting people from your past, the consequences of past experiences leading to situations you are now enduring etc. to be easily established in the game world.
A final pragmatic example, lets look at one of the most basic options for creating your own character. Are they male or female? Now look at how much intricate dialogue exists in the Witcher 3. How are you supposed to have cut-scenes of this depth and beauty if you don't know before hand if the protagonist is male or female? Would you expect developers to spend time and resources doing all of the protagonist dialogue then simply do it all again in the other male/female voice just because you might pick a different gender during character creation? Although clearly possible, this is not feasible. For the main story you would literally have to make the entire series of cut-scenes twice with different dialogue in each, since a male is going to interact differently with Ciri, for example, than a female.
So here on the other side of the debate I maintain that it would be best that if they make another Witcher game to not allow for unique character creation as you suggest.