Why is this a problem though? Does anyone actually argue that the death penalty is unconstitutional by that logic? The only constitutional arguments I generally hear have to do with the “cruel and unusual punishment” part of the fifth amendment. I would also argue that loss of limb is meant as a catch all for any bodily injury that doesn’t cause death. But it seems pretty straight forward to me, either you believe an ‘evolving standards’ principle is applicable to the constitution, in which case you can argue against the death penalty assuming you can make a strong case for the current standards - or you don’t believe in it in which case there’s just no way to argue that the death penalty (or loss of limb) is unconstitutional.
Seems to me that life and limb are inexorably intwined and you can’t make separate arguments about them. Further, I don’t see any evidence that that kind of argument is being made by anyone. It would seem to depend on people thinking amputation is somehow worse than execution, which I can’t imagine anyone taking seriously.
I would also note that whether or not the constitution allows loss of life or limb, it certainly doesn’t mandate or guarantee it. A federal law could abolish it without needing to have a judicial ruling end it.