Well, we should probably stray from talking about the matter in the specific, and instead talk about it more in the abstract. Discussing child-killing is against forum rules, which is why I've been trying to stick the the NPC side of things while admonishing that children must be immortal if they are included.
These exist already... the emperor's private quarters, the orrery room door (in the game as sold), the emperor's robe respectively. There are other examples of this, as well. But I am accepting of them because there is so much else to do.
That's very true. However, these kind of things are fairly... foreign to TES, at least the vision of TES spawned by Redguard and Morrowind. Daggerfall did have some of that element with houses that were for filler purposes only, but not a lot. They do not need to be encouraged, either. Nothing should be off-limits to you simply because the game mechanics or developers will it to be so.
The actual lack of an Emperor's quarters is a huge oversight on the part of the developers. Not having access to the entirety of White-Gold Tower in terms of what is already there is because of certain scripting decisions on behalf of the final Thieves Guild questline. However, workarounds were (and always are) possible and preferable. In the Thieves Guild example, scripts could have been written up that didn't rely excessively on the interior data, meaning the player could enter and exit the specified interior cells without setting off scripts for the quest in question. Or, if that wasn't possible (big if), an exact replica of those interior cells could have been set apart to be used and referenced in the Thieves Guild quest, allowing the player to still mess around in the real interior cells without screwing up potential future quest data.
The Orrery, it is suspected, was intended to make it into the vanilla game, and it was cut and later packaged into DLC due to time constraints. So it was not the intention of the developers in this case to create a door to nowhere.
The robes are just another example of an awkward design concept that I still have yet to understand; not being able to loot things to the fullest. If the player wishes to loot the robe from the Emperor's body, they should be able to do so. It would be even more interesting if the robe had been scripted to cause a massive negative reaction while wearing it, a la the Ordinators attacking you for wearing Ordinator armor in Morrowind. Rather like me killing off hordes of Dremora, only to be allowed to loot their weapon and shield, not their armor. Same thing for killing off Legion Palace Guards. Very damaging to immersion. It makes absolutely no sense to not allow the armor to be looted. Any objection that could be brought up regarding it can be worked around. Unbalances the economy? Then adjust prices to factor it in. Too good of armor for the player to be using? Then make it's wearer tough and durable; if I see some nice shiny armor, I decide killing for that shiny armor is acceptable, and I do manage to successfully kill, then that armor should be my duly earned reward.
So while, yes, these limitations did exist in Oblivion, they need to be culled.
However, if Burd was non-essential he could be killed by the player at any point before the mission started breaking the mission. Or did you want there to be a contingency to replace him? And then a contingency to replace that one if he or she should die? I'm not sure what exactly you want to have happen if Burd should die. Should it be a hard failure; time to reload? I'd prefer a temporarily invincible partner AI; however, unlike Burd, one that lends the illusion of struggling throughout the fight, without falling down at all. Instead, how about if he gets hit for a massive amount of damage he requests to wait for a minute to catch his breath after the combat is concluded? After the important plot points I'd like him to no longer be essential.
Well, for one, they need to stop designing quests that hard-nosedly require a specific NPC to be alive for them. Side-quests that involve people specifically are understandable, as is the consequence that if you knock someone off, the quest cannot be started or go on. But for major quests, or even quests that are not central to the people they flag essential, it's a flawed design. Working around Burd's death is easy. If he were to die before the gate quest, then he would be replaced with a generic Bruma soldier, who would have appropriate dialogue to spur the men, perhaps honor their fallen captain, and pledge to charge into the gate with you. Problem solved. Burd no longer essential. Similarly, if Burd should be alive when starting the quest, but Burd should die during the quest, then Burd is dead. Why should that completely stifle any outcome? If Burd dies, you close the gate yourself, report his death to the men outside, and continue on your merry way down the quest-path.
In Morrowind, no NPC was immortal, and the only NPCs that were essential were the barest minimum that were needed to complete the MQ. If you killed one of them unaware (as the game didn't point them out for you), a dialogue box would appear telling you that you had doomed yourself and the MQ, and that it was your choice to continue in that doomed world or reload a save. The great thing was, even if you chose to continue in your "doomed" world, there was still a backpath, an avenue by which, even through killing someone (or multiple someones) insanely important to the story's continuity, you could still work your way through the MQ in a logical fashion. It was hard to stumble upon, but the fact remained that the Morrowind MQ could be done without any
real essential NPCs at all.
The point is, a well-designed and fleshed out quest doesn't need to rely on NPCs for the quest to be finished, unless the NPCs in question are the absolute core of the quest. And then, you just have to live with the consequences of a quest that can't be completed if you kill them (think side-quests). The vast majority of quests do not have NPCs as their direct object, especially regarding factions or MQ. And even if they may involve NPCs in the pursuit of the direct object, the NPCs themselves are not central to the pursuit of the direct object. Your primary goal in the Bruma Gate quest is to close the gate. Teaching the guards how to accomplish it is a secondary notion. If none of them survive to be taught, then that's harsh life. The game and the world still continue.
It's also very true that what is immersion-breaking and what isn't is very much a matter of opinion and which side of the coin you fall on. What NPCs can do is very limited, and what can be portrayed is very limited. I've geared a lot of suggestions towards trying to give life to NPCs via more dialogue, random encounters, and just general tighter AI, but at the end of the day there's a lot be desired of what a video game can portray of people, especially when it's ambitiously attempting to portray 800+ people. However, there are also a whole host of things that can be and are exhibited via the game regarding NPCs. One of those things happens to be the realities of getting damaged and how that damage affects them by injuring and eventually killing those NPCs. And to have a whole subsection of NPCs that are essentially-flagged for whatever reason who are outside of those very tangible rules is breaking to immersion. It may be presented in the form of "just killing," but it strikes more at the heart of the mission statement and philosophy of TES. Open world, do whatever you can conceive.
If TES:V ships with essentially-flagged NPCs, I will still undoubtedly buy it, and I will still undoubtedly enjoy it. It's not too big an issue for that. But it is something I would prefer not make a comeback.