has been made unable to cause further harm

I feel like this is where I diverge, I dont feel like any prison solution is permanent or without flaw, I can very easily imagine scenarios where a permanently incarcerated person could not be that anymore, and continue or be emboldened by their release. I do agree that a government free of class conceptions and the trappings of a State wouldnt need to or would have very little reason for a death penalty. We are very far from those times.

Also quick edit, while I think there is utility in the death penalty, I do also think theres not a lot of good solutions for carrying it out in a humane way that shows care rather than the barbarity of it. And also whether or not the barbarity of taking someones life should be exposed for how brutal it is or should be softened to reflect a better nature of carrying it out.

Though whether they are able to cause further harm or not from inside of a prison cell remains to be seen. Look at Nalvalny in Russia, even completely powerless and imprisoned, the west still tried to use him as a tool to manipulate the Russian people and forment colour revolution. A politician in China could easily end up with the same sort of support from the west if they seem like a good puppet (and someone extremely corrupt certainly would make for a great puppet).