I don't see why everyone is so skeptical here, Assassin's Creed managed it perfectly well even with The Frontier and Rome and Constantinople being pretty large. For a game of The Witcher 3's size, or Skyrim's size, you just have to scale that up. The Assassin's Creed team did it by splitting the game into area chunks and having adjacent chunks automatically load whenever you were inside one, so the loading actually happened before you ever got to an area as an anticipatory thing. The only issue here I can see would be with the processing power required due to the graphical detail in the new Witcher game, and that's easily remedied by having smaller chunks to load or by the player having higher RAM, which is much more common in this day and age anyway.
See? No problem and anyone complaining about it should actually take the time to do their research before commenting.