Your substitution is inaccurate. The rulings effectively mean that the death penalty is cruel and unusual except in certain, very specific circumstances. Only the most extreme crimes qualify. Even the rape of a child is not extreme enough unless done multiple times. It might seem applicable to only a few cases, but that is only because few states attempt to apply the death penalty except in the most extreme circumstances.
For example, in 1980 they ruled that it could only be applied only if it involves a precise and aggravating factor[0].
Louisiana had a law that made the rape of a child punishable by death and SCOTUS overturned that law in 2008[1].
SCOTUS has consistent narrowed the crimes punishable by death in the past 50 years using the Eighth Amendment as a justification.
[0] https://en.wikipedia.org/wiki/Capital_punishment_in_the_Unit...
[1] https://en.wikipedia.org/wiki/Kennedy_v._Louisiana