Hard boss fights are story breakers. If the point of this game was fighting, they might not be game breakers; you might retry and retry and retry and retry a fight until eventually you got lucky. But the point of this game is story. Every time the player character dies is immersion-breaking; every time the player character dies the designers have failed.
I've personally never got past the Kayran in Witcher II. Yes, I could retry and retry and retry and retry and eventually I'd get lucky, but that isn't fun and it isn't interesting and I have better things to do. The Kayran in the Witcher II is in my opinion a classic example of poor game design: there's no alternative path round it, the fight is out of scale with the fights you've gone through so far, and the mechanics are different from other fights in the game (the dragon bridge fight in the prologue was another example of this).
No one game can appeal to every sort of gamer. CD Projekt are exceptionally good at story: they have an audience of loyal fans who buy their games for the story. If they wreck the story with big boss fights in order to try to attract gamers from another genre, all they'll do is produce a game that pleases no-one.
So no. It's fine to have a ludicrously difficult difficulty setting for people who actually don't give a shit about immersion or story, but there also needs to be a difficulty setting on which every player who's paying attention and has learned the mechanisms established in previous, easier fights can reasonably expect to get through without flatlining.
Because, in real life, people who flatline don't get to finish the story.
