So under your theory, who decides what is cruel and unusual punishment under the constituition?
The States decide. If there appear to be abuses due to changing public opinion or technology, the people of the State should change their laws. If the Nation feels the necessity to correct the action of a minority of the States, there should be an Amendment to the Constitution.
The Supreme Court should interpret the meaning of the Constitution, not engage in the equivalent of amending the Constitution.
Evidently it is not SCOTUS. They refused to hear the case of Terry Schiavo. Starving and dehydrating any person to death would fall under the rubrick of "cruel and unusual", no?