Again, I am aware that there is no core technological or logistical issue. The issue is purely societal. Yes, I can get behind enabling people to specify policies on what to do when untreatable or mental diseases kick in.

The death penalty does not exist to reduce the suffering of the convicted, but to get rid of them. The true issue with the death penalty is that it can't be graduated (except by adding "cruel and unusual punishment") and it can't be undone. Prison sentences can be legally challenged and the innocent can be freed early.

There is a real slippery slope here: what length of prison sentence is considered to be worse than the death penalty? An additional thing to consider is that many countries without the death sentence actually don't impose true life sentences, but very longish ones (upwards from 20 years). Confinement for life is for those irredeemly judged to be a threat to society after their sentence. Compared to that, many death row inmates actually spend decades fighting their sentence. They could end it at any time if they wanted.