As far as I'm concerned they can "enjoy" their time in their cells as much as they like. That's a non sequitir in the debate.

It actually is very relevant.

The premise of putting a man behind bars for the rest of his life is the implication that the man will suffer great mental anguish. That premise has been challenged in the field of study of the criminal mind.

If you really want to get some good insights, read the works of Dr. Samuel Yochelson and Dr. Stanton E. Samenow. They have been major myth-busters in the field.