>saw this in memes a million times over the years
>turns out in most cases you actually can still complete the story even if you kill a so-called "essential NPC"
What is the point of this then if it doesn't actually signify a "doomed world"?

what meme

>>712904541 (OP)
It just means the events of the game became non-canon. TES are pretty specific to who lives or dies between games.

Some characters have to progress a main quest through a conversation, and if they are dead then their conversation can't happen and so (without using glitches or guides) the main quest can't be finished.

>>712904541 (OP)
You can still complete the story, but you're technically not the Nerevarine reincarnate anymore. That is until you do defeat Dagoth Ur, then Azura just pulls a plot convenience out of her ass and says that you are.

In Oblivion they added a tag that the devs could flip and main quest critical NPCs would become immortal.
In Morrowind you had this popup instead. If you see that message you can't complete the main quest.
You can beat the final boss by using the back door, or by abusing alchemy to turn yourself into a Skooma infused God with the speed to clip through walls and the strength to delete any enemy from the game in one jab (you also destroy the weapon you had equipped in the same attack, you're that beefed)

>>712904541 (OP)
it means that you can't do it without a guide, cheese or insane guess work

>>712904541 (OP)
>in most cases you actually can still complete the story
only with the backdoor non-nerevarine route which is not canon and requires meta knowledge