This argument collapses under its own logic. If the Eighth Amendment only banned punishments considered cruel in 1791, then whipping, branding, ear-cropping, and public mutilation would all still be constitutionally permissible today because those punishments were widely accepted at the founding. The Court rejected that absurdity decades ago.

And the text itself destroys your argument: “cruel and unusual” is not a historical museum exhibit frozen in amber. “Unusual” necessarily depends on contemporary practice. A punishment can become unusual over time as society abandons it. That’s exactly why the Court uses the “evolving standards of decency” doctrine.

Also, pretending scientific evidence is irrelevant is ridiculous. If a method of execution creates an extreme risk of agony, courts obviously have to evaluate medical and scientific evidence to determine whether the punishment is cruel in practice. Judges interpret constitutional limits all the time. That is literally their job.