Give me an example.
An NPC assassin sneaks up on my sleeping 5th level Fighter. He puts his sword to my neck and pushes hard.
Auto-kill in your game, or do the rules apply?
By rules I mean: The DM narrates the action, surprise is determined, then positioning, then initiative, then turns taken in order.
Well, for starters, this example could pnly happen in my game after a series of serious failures by the PCs, because I'm not a jerk GM.
That said, if I thought the outcome of this wasn't uncertain, then, yup, better grab a new character sheet. The fundamental play loop of the game remains unchanged: it is the GM's authority to decide if player actions declarations succeed, fail, or are uncertain. The more detailed resolution mechanics for combat do not change this. Granted, the rules atrongly lean towards treating any violence as uncertain, largely because such a huge chunk of thise rules deal with uncertain violence resolution, but it doesn't remove the GM's ability to decide.
In other words, nothing about combat overrides the GM's authority to declare success, failure, or uncertainty of an action. This remains true even inside the combat loop. Principled play, by which I mean the GM follows clear principles of play so everyone at the table understands and can anticipate rulings, are critical in a game like D&D. Traditionally, and sadly, nost if not all of these has been rooted in the rules -- either taking a rules as a must and always using it or using a gap in the rule as allowing whatever isn't proscribed, again always. 5e makes this hard because it leans heavily on GM decides as it's core mechanic. This leaves many in a quandry because it doesn't provide a strong principle set via the rules as written. Therefore, people take what rules there are and fashion them into ironclad principle sets, which, if challenged or if a different approach is presented, feel like your principles are under attack. Like how combat rules must be used if hp are involved are justified by the detailed rules, forgetting that thise rules nest inside the broader set where GM's always retain the authority to decide outcome.
To be clear, it's 100% awesome if you always rule anything like combat is uncertain and use the combat rules. That's a very principled approach. It, however, isn't the only one, or best one. The best one is, of course, the one that maximizes fun at your table. So, best for you is as good as it gets. But, using the combat rules is not required by the ruleset of 5e unless the GM determines uncertainty (a generally wise determination), and there are other, equally principled approaches that work.
Finally, I'd like to, as is my wont, to caution against agruments from jerkdom. It seems common to challenge a different approach by postulating a situation caused by jerks, like the above GM having an NPC assassinating a PC by fiat. This may seem to support your argument because this can't happen with your approach, but that's a false appearance. For starters, you've assumed that the other poster is a complete jerk and doesn't have other principles that would prevent the situation from occurring. Second, your method doesn't really prevent a different jerk from breaking it either -- for example you can have a 20th level wizard NPC cast meteor swarm on the same PC from a far distance, and employing the combat rules diesn't change tge outcome. If this immediately raises a "I wouldn't do that, it's a jerk move," defense, then don't postulate jerks on other posters.
