The other humans did have the save power.
Asgore was taking it easier on Frisk because he didn’t want to take the final soul. As Toriel points out, he was desperately hoping another human never fell. Asgore gave us chance after chance to back out, and said if there’s even the SMALLEST thing we didn’t do, we should go do that. He doesn’t want to fight us. And when he loses, he asks us to just take his soul.
He wants to lose this fight.
He probably was a lot less hopeless against the other humans, and Undyne confirmed Asgore can actually dodge.
And Asgore can destroy the mercy button, maybe he eventually just destroyed the FIGHT button for the other humans or something
Flowey was made from the human souls and tried to take ours when we first showed up. This heavily implies he was made after all the other humans already died, so the other humans would have had the ability because Flowey didn’t exist during their time.
By the way, I used to actually think determination was only in the soul and losing the soul meant you die for humans just like monsters.
It was a part of a theory I made that the six humans came back to life in True Pacifist after Asriel restored the SOULs (thus explaining why the coffins opened.)
Here’s some words from the person who debunked that old theory of mine.
Jacky720: Undyne clearly has a high level of bodily determination. High enough to melt and reform, regaining her full HP like the Amalgamates on a lower level. But, her SOUL does not persist, even for a second. We have a contradiction. Boss Monsters like Toriel and Asgore have the opposite properties, and we know both are the result of determination: I believe this is what gives their SOULs the strength to persist after death.
I already hear your rebuttal. "But Undyne could have gotten determination from the ice cream!" For this, I have two counterpoints. First, Alphys has no reason to put the super-dangerous-super-experimental-only-for-use-on-people-already-at-death's-door substance in the ice cream for her crush. That part never checked out to me. Second, we know that determination generated by Undyne herself was still at least part of it- that's why she only successfully transforms on Genocide, after watching you attack a defenseless child and feeling the thousands of damage from your hate (or, LOVE). Under your model, at least that part of the DT had to come from the SOUL... which doesn't persist.
Now back to my own words, using what they said as a base. If determination is based in the SOUL, and we know for a fact determination is what allows Boss Monsters to persist, and monsters having too much determination causes melting, either Undyne has more determination and her soul should persist for a few seconds like boss monsters do, or she has less and she shouldn’t melt.
She melts, so she has more determination than Toriel. But her SOUL doesn’t persist, which implies her SOUL has less determination. This implies something other than the SOUL is where the extra determination is.