But the hit point issue is one of "end the skill challenge in a success," where "success" is being linked to "killing an opponent."
Yes, it's a bit of a strained metaphor because a dead opponent isn't necessarily the only victorious end-state (surrender also works*), but you don't get to dead without going through the HP buffer.** Just in that fashion, you don't get to a victorious end-state in a skill challenge until you go through the success buffer.
If the skill challenge being undertaken is a negotiation, the players can decide at any point to toss over the table, out blades, and extract their demands at swordpoint. I'd hardly count that as a victory in the skill challenge, though! It's, in fact, a failure in the skill challenge, which the part now has to react to. Similarly, players can exit a combat challenge early with a failure by surrendering***.
* But, given that HP represent morale, among other things, and that players may declare an opponent to not actually be dead upon striking the blow that takes the target to 0HP, narrating a "killing blow" as "the enemy surrenders" looks acceptable to me.
** Or using a Save-or-Die - but I don't really care for those for various reasons, so we'll leave them aside.
*** Hah, hah, HAH! No, really, I heard it happened, like, once.