The problem relies in the fact that players (a large part) want both to be able to kill plot important NPCs and be able to finish the quests in the original way.
I find annoying to have unkillable npcs not because they're way over your level (that would imply that the npc level is adjusted to match the player's level) but because they are key to the plot, just like I don't like the "this quest is very important"/"you can explore all the map of the world, chat with every npc, loot any single chest or container, pickpocket the full population and the quest will still be there to you to complete".
Choices should bring consequences, something that challenge the argument about verosimilitude. How can be verosimile to have the same identical outcome wheter you kill a key npc or don't?
In my opinion instead of consuming energies in creating multiple ways to reach a same outcome, developers shoul concentrare and focus on create equally satisfying, engaging, and interesting different outcomes.
Alsa that is a dream. Producers know that players want both the wine barrel full and the wife drunk (is an old italian say "volere la moglie ubriaca e la botte piena" that mean to want everything even things that are completely opposite).
