I agree. It's crazy to me how an entire location within the story is just not accessible depending on your choices.
I beat it two night ago and immediate started another game making different choices. But on top of that, it's not just "choose the other side", as there are combinations of smaller choices within the larger ones that influence the outcome of the game. With the way the game ends, I assume Witcher 3 will read your Witcher 2 save file and adjust accordingly, Mass Effect style. I can't imagine they'll go the route of being accessible to people not familiar with the story again.