You're trotting the goalpost back, whether you mean to or not. Death sentences represent a substantial liability to the state, because killing someone on the basis of an accusation that turns out to be false is a danger, one that is worse with a faster turnaround to these events. The state is happy to have false convictions, so long as they don't cause the boat to rock.

There are surely even true-blue people like you who help promote these measures on some high-minded basis of minimizing infringement on the rights of citizens, but what has been most consistently demonstrated by the state is that if lofty liberal principles come into conflict with material interest, the latter wins. If the state gave a shit about "rights" among the general population, we wouldn't have hundreds of de-facto summary executions per year doled out by the cops (this making allowances for actual violent conflict, since the total number of killings is past 1000)

The simple fact of the matter is that these long periods of internment work as an alibi for the state and, most egregiously, allows for the killing to be carried out with substantially less ado. This is a well-tested method that is notoriously used for the crimes of federal agencies wherein they wait a few decades and then publish what they did once things have cooled down, defusing the liability of another discovery like Cointelpro.