I'm complaining more about the motherfu-ARGLEBLARGLE-sential NPCs".
Actually, I think the immortal NPC is not just protection against the player, it's protection against themselves. It's incredibly easy for NPCs to get into a fight even when you didn't do anything wrong. They walk around all over the place, too, straight into the morning breath of some dragon that landed on the local Inn to loudly ask for directions. It's also rather easy to accidentally kill the wrong NPC when you're yelling directions at the poor lost dragon, so immortality against other NPCs only wouldn't work. Even accidentally dropping a dagger can spark a fight.
NPCs in Morrowind hardly moved about or faced random monster attacks, if anything happened it was the player's fault.
Skyrim's solution is a bit crude, but it gets the job done. It's a bit like how the first 3D games looked horrible compared to 2D sprites. (And sometimes still do) Once the technology is perfected, we'll have quests that adapt to replace missing actors.
The thing is, they've already shown that they can have NPCs that aren't going to be affected by this with the companions, who can only be killed by players, and by incredible streaks of bad luck with AoE attacks and damage over time. I have no doubt that they could make NPCs that are important to major quests functionally immortal to everything but the player, with not very much effort. The only change that would need to be made would be something that prevents those certain NPCs from taking damage from any source other than the player while in their healing trance state. How is this not a better solution than just slapping immortality on every character associated with any quest longer than "go here, kill these bandits"?
In regards to the player accidentally killing an NPC: that is part of the challenge. You can't blindly spray fireballs or hack away at a crowd of people unless you're willing to face the consequences of your actions. If you accidentally kill a quest-central NPC, well, then you have a choice. You can reload (if you had any sense, your last save probably isn't too long ago; big fight sequences are telegraphed pretty obviously ahead of time), or you can keep playing on the character, accepting that the quest won't be available, if you prefer to RP like that.
Basically, anything that is taking power away from the player (and in some cases, from random chance) in order to prevent the >player< from "screwing up" (In other words, deviating from the linear plot of the quests) >their< game is going to be negative, so long as it isn't something where it is preventing an unavoidable bug, etc. The best RPG worlds have always been the ones in which the player is able to drastically alter the state of affairs, or alternately a set state world with a perfectly repeatable, linear storyline. Skyrim (and, admittedly, early entries in the series) fall into the gap, where they're too loose-ended to make a proper linear story, and the sense of urgency surrounding the main quest feels absurd, but too restrictive to be a truly freeform world.
If they really want to give players freedom, they need to ditch the BS about Radiant AI, drop the handholding measures, and get rid of the repetitive quests and writing. If I want to murder every Jarl, Ulfric, and all of the Imperial officers, and watch Skyrim descend into chaos, I should be able to do that. If I want to hone my diplomatic skills working out deals between individuals, between villages and bandit groups, etc., and then negotiate a peace between the Imperials and Stormcloaks, turning them both against the Thalmor, I should be able to do that. If I want to become a wise old hermit living deep in the wilderness, with new adventurers seeking advice from me, I should be able to do that. If Bethesda doesn't want to follow up claims of opened, emergent worlds and NPCs that adapt to change, they need to stop harping on about how great Radiant AI supposedly is and work on core mechanics and writing (ye gog, the writing) to make what we do have a better game.