I don't see why the devs should have to accommodate your autistic killing sprees, making the game finishable no matter who you kill is a cop-out itself.
It doesn't have to be a killing spree, it might just be a single NPC that the player wants dead for whatever reason — could be a major player in the game world, or some random hobo who just happens to play a minor part in the main quest. Non-killable NPCs aren't usually problem in games with a set protagonist and/or decent writing, but with blank-slate protagonists it can become annoying or outright infuriating (see: any recent Bethesda game).
I thought the best thing about Morrowind's approach was that you could fail the main quest and
still finish the game. You just needed to delve pretty deep into the game to figure out the backdoor.