Like I said, we aren’t there yet, and may never get there, any hypothetical death penalty cases that would meet that standard would have to be decades if not centuries into the future with the benefit of all the scientific advances that come to us in that time. It’s probably more likely that we’ll have a 100% effective way to treat and rehabilitate people by then, but if we ever do reach a point where we have hit a complete dead end in psychology/neurology, in some hypothetical far-off, post-technological-singularity future where we can definitively say that we have exhausted every possible avenue for rehabilitation, then we can consider the death penalty.
And I disagree that it can only be applied to current and immediate threats, because people who have shown that they are likely to harm people again would then have to spend the rest of their lives restrained and/or in solitary confinement if they’re unable to be rehabilitated, and arguably I think death may be a more humane sentence.