Not only is the death penalty irreversible, it's also a get-out. Solitary confinement for life, no chance of parole, subsistence gruel only, no entertainment, exercise or work allowed - a harsher punishment in my view and ultimately reversible. The only downside would be if they were pardoned, you would have one seriously screwed-up person out there (better that than dead?).
No reason for imprisonment to be so expensive either - the expense only comes because we keep letting people out of their cells for visits etc and hence need multiple layers of security!