You’ve ignored the caveat that there is no reasonably effective defense. Pointed crossbows don’t inhibit or prevent an effective defense.
Last post, and then I'll be done with this.
How would the player know that? I mean, to begin with, how did you the DM know that a pointed crossbow didn't inhibit or prevent an effective defense?
Suppose that the pointed crossbow was pushed up against the PC's back? Would that now "inhibit or prevent" an effective defense? Would this be equivalent to the "knife to the throat" situation now?
What if the crossbow was six inches away? What about a foot away? How far back does it have to be before the player can make a judge whether resisting is suicidal?
How many kobolds are required to grab a PC so that they negate the PC's ability to effectively defend and allow one of their number to gut the PC without an effective defense? You did say, "swarms of ankle-biters like goblins and kobolds who might try to overwhelm and drag you down." How many is a swarm, that I might know ahead of time as a player when I'm automatically doomed and have no defense? For example, if any one of them win a contested athletic check with me in combat, does that mean I now have a knife to my throat and so cannot effectively defend myself?
Why would anyone make normal attacks when they could bypass hitpoints in this manner?
How big does the dragons neck need to be before my plan to put a dagger to its throat is invalidated, and I would have been better off never attempting it?
If a simple Dexterity check is enough to achieve knife at the throat position as you assert, why does anyone need an attack bonus, AC, or damage bonus in your game? Won't basically any creature with a good Stealth check be able to kill anything about its own size, fully negating any defenses that the target has in AC or hit points? How does any player in your game survive such an onslaught save by your grace? Or how does a player know when a monster is protected by your grace so that they must resort to the risky strategy of the rules?
Your failure to imagine a good use for the rule doesn’t mean there isn’t one. Your insistence that all uses of the rule are DM-screwjobs doesn’t mean they are.
At this point, all I need to do is point to post #78 and say, "Those are your ideas regarding what good uses of the rule are."
Severing a limb is a big deal and a massive advantage in combat. So I’m okay with making it hard to do.
Hard to do? So if I take disadvantage on an attack roll, and succeed, I can cut off my opponent's leg by doing more than their CON in damage? Have you play tested this?