That's if you scale HP and damage, then you would eventually get to the point where you have enough damage to one-shot a lower-level tough enemy.
For whatever reason, they didn't really do that. They went with scaling HP primarily, and then minor scaling of attack bonuses and damage and numbers of attacks. I guess it kind of gets there, if you consider three attacks (with power attack balancing out the higher attack bonus) to count as one-shotting. It's still dead in one turn, at least.
If they'd gone with AC scaling, though, then they wouldn't have needed to give the ogre nearly as many HP in the first place. Instead of it having AC 11 and 59hp, they could have given it AC 19 and 29hp, and a high-level PC with +20 to hit would be able to drop it quickly in 1-2 attacks.
I guess that's just a matter of preference, though. In terms of game balance and flow, it doesn't really matter whether you hit it six times for damage, or only three times for damage because the other three attacks failed to hurt it. It's just a weird mental image that you need to actually hit with so many attacks before something will fall down.
I sort of answered this already, but I think I understand your point better.
You're talking about a high level character being able to one-shot a lower level character.
So to begin with, the mechanics are human(oid) centric. So a one-shot killing blow is something you can measure best against that standard. Such as the commoner NPC. So there are a lot of monsters, traps, and even a critical hit from another 1st level character that can one-shot pretty much any person in the world except for those with more extensive combat training (that is, a character of higher than 1st level).
As characters get to be higher levels, there are two primary mechanics in play for increasing martial damage:
Scaling damage, like the rogue's sneak attack, and additional attacks, like the fighter. The barbarian and Ranger (Hunter) have a bit of both. Rogues have a higher chance of a one-shot kill. But the fighter-types have a better overall chance in combat.
I would recommend adding the System Shock rule (DMG pg273) which gives you a 30% chance of a one-shot kill if you do at least half their hit point maximum in a single shot.
As I've mentioned elsewhere, I do have mechanics for one-shot kills, and I think it largely addresses your concern, because it works best against targets of lower level than you.
Called Shot: You attack with disadvantage. If you already have disadvantage, you cannot attempt a called shot. If your attack hits, the target gets a saving throw with a DC of 8 + your attack modifier. Many of the called shots allow the target to make an opportunity attack against you if your called shot fails.
·
Sever.* You can attempt to decapitate your target, or amputate a limb. The target must make a Dexterity or Constitution saving throw (their choice).
There is no special effect unless they fail their saving throw by 10 or more, in which case the limb is severed, throat slit, etc. If your attack is a critical hit, they only have to fail their save by 5 or more to lose the limb.
Certain creatures are more susceptible to this attack, such as hydras, ropers, and most creatures with tentacles, in which case the attack is successful if they fail their saving throw.
The most important warning I can give about any rules like this is that if you have the option for doing it, so does the monster. So at some point, this
will happen to a PC. It's just a matter of simple math. If the chance is 1%, then along the course of several sessions you'll roll that 1 in 100 chance. So you want it to be hard, and you want it to be something that most monsters won't attempt because they have better options.