I think you outline most arguments in your OP and address them.
It is a difficult question society needs to ask itself.

I don't think that death penalty acts as an efficient deterrent. Spending 30 or 40 years in jail, or even a life sentence, seems like a very good motivation to not commit a crime. I don't think that simply by adding more punishment we get better efficiency and I think statistics back this up.

Losing a person in our society, even if it is a criminal, still impacts us negatively. We benefit from every mind that is alive today, if for nothing else, to tell their story.

There is one type of situation where I am thinking that death penalty might be applicable and that is if a person in question really is so dangerous and competent to hurt others even if sent to high security prison. These situations are so rare however, so I don't think that this makes any significant impact on the decision.