I have to be honest and say that I genuinely have never understood this attitude. Why is it at all an issue that plot-important NPCs are invulnerable? And how is it a meaningful improvement when every NPC is killable? Like, I'm indifferent to that as a design choice but I have seen so many people in favour of making every NPC killable and I just want to understand why. What is meaningfully gained in terms of experiencing gameplay or story if you can kill a plot-important NPC and potentially block off the quest? Especially if it's the main quest and not an optional side quest that can just be checked off as failed.
And regarding the example you gave, a computer game isn't a live game at the table, where a DM can potentially pivot and make their story work in spite of a dead NPC. A computer game is far more limited and I can't fault a dev for making their lives easier by letting themselves not have to accound for every bit of random death and destruction that the player can cause. Sure it's nice to imagine the possible creative results killing important NPCs could give, but realistically it would just leave you screwed out of completing the main game and at that point, what even IS the point?
It's not important to be able (no, "to have the OPTION to TRY", being able is a different thing) to kill everyone because it's a nice to do.
It's important because it's part of the systemic design that creates the illusion of the artificial world that is consistent with its own rules.
Every time the players are denied an action (no matter how inconvenient) that they should be reasonably be able to attempt or every time a game fails to acknowledge something particularly out of the ordinary that they are doing, the illusion cracks a bit. More and more cracks piling up can break the spell.
Which is why when a game goes out of its way to open doors to the player (the option to attempt killing anyone, the option to talk to non-plot relevant characters) and be reactive to his actions (getting angry or suspicious if something is stolen, commenting if the character is naked or dressed inappropriately, acknowledging if the player killed or helped someone else, having a distinct behavior if the MC is male, female, elf or dwarf, etc) the illusion is reinforced and the player feels more rewarded for the time invested in the simulation.
